| perl6 Synopsis 5: Regexes and Rules |
Synopsis 5: Regexes and Rules
Damian Conway <damian@conway.org> and Allison Randal <al@shadowed.net>
Maintainer: Patrick Michaud <pmichaud@pobox.com> and
Larry Wall <larry@wall.org>
Date: 24 Jun 2002
Last Modified: 8 Oct 2008
Number: 5
Version: 85
This document summarizes Apocalypse 5, which is about the new regex syntax. We now try to call them regex rather than "regular expressions" because they haven't been regular expressions for a long time, and we think the popular term "regex" is in the process of becoming a technical term with a precise meaning of: "something you do pattern matching with, kinda like a regular expression". On the other hand, one of the purposes of the redesign is to make portions of our patterns more amenable to analysis under traditional regular expression and parser semantics, and that involves making careful distinctions between which parts of our patterns and grammars are to be treated as declarative, and which parts as procedural.
In any case, when referring to recursive patterns within a grammar, the terms rule and token are generally preferred over regex.
The underlying match result object is now available as the $/ variable, which is implicitly lexically scoped. All user access to the most recent match is through this variable, even when it doesn't look like it. The individual capture variables (such as $0, $1, etc.) are just elements of $/.
By the way, unlike in Perl 5, the numbered capture variables now start at $0 instead of $1. See below.
The following regex features use the same syntax as in Perl 5:
While the syntax of | does not change, the default semantics do change slightly. We are attempting to concoct a pleasing mixture of declarative and procedural matching so that we can have the best of both. In short, you need not write your own tokener for a grammar because Perl will write one for you. See the section below on "Longest-token matching".
Unlike traditional regular expressions, Perl 6 does not require you to memorize an arbitrary list of metacharacters. Instead it classifies characters by a simple rule. All glyphs (graphemes) whose base characters are either the underscore (_) or have a Unicode classification beginning with 'L' (i.e. letters) or 'N' (i.e. numbers) are always literal (i.e. self-matching) in regexes. They must be escaped with a \ to make them metasyntactic (in which case that single alphanumeric character is itself metasyntactic, but any immediately following alphanumeric character is not).
All other glyphs--including whitespace--are exactly the opposite: they are always considered metasyntactic (i.e. non-self-matching) and must be escaped or quoted to make them literal. As is traditional, they may be individually escaped with \, but in Perl 6 they may be also quoted as follows.
Sequences of one or more glyphs of either type (i.e. any glyphs at all) may be made literal by placing them inside single quotes. (Double quotes are also allowed, with the same interpolative semantics as the current language in which the regex is lexically embedded.) Quotes create a quantifiable atom, so while
moose*
quantifies only the 'e' and matches "mooseee", saying
'moose'*
quantifies the whole string and would match "moosemoose".
Here is a table that summarizes the distinctions:
Alphanumerics Non-alphanumerics Mixed Literal glyphs a 1 _ \* \$ \. \\ \' K\-9\! Metasyntax \a \1 \_ * $ . \ ' \K-\9! Quoted glyphs 'a' '1' '_' '*' '$' '.' '\\' '\'' 'K-9!'
In other words, identifier glyphs are literal (or metasyntactic when escaped), non-identifier glyphs are metasyntactic (or literal when escaped), and single quotes make everything inside them literal.
Note, however, that not all non-identifier glyphs are currently meaningful as metasyntax in Perl 6 regexes (e.g. \1 \_ - !). It is more accurate to say that all unescaped non-identifier glyphs are potential metasyntax, and reserved for future use. If you use such a sequence, a helpful compile-time error is issued indicating that you either need to quote the sequence or define a new operator to recognize it.
/x) is no longer required...it's the default. (In fact, it's pretty much mandatory--the only way to get back to the old syntax is with the :Perl5/:P5 modifier.)/s or /m modifiers (changes to the meta-characters replace them - see below)./e evaluation modifier on substitutions; instead use:
s/pattern/{ doit() }/
or:
s[pattern] = doit()
Instead of /ee say:
s/pattern/{ eval doit() }/
or:
s[pattern] = eval doit()
m:g:i/\s* (\w*) \s* ,?/;
Every modifier must start with its own colon. The delimiter must be separated from the final modifier by whitespace if it would otherwise be taken as an argument to the preceding modifier (which is true if and only if the next character is a left parenthesis.)
:i :ignorecase
:a :ignoreaccent
:g :global
:i (or :ignorecase) modifier causes case distinctions to be ignored in its lexical scope, but not in its dynamic scope. That is, subrules always use their own case settings.
The :ii (or :samecase) variant may be used on a substitution to change the substituted string to the same case pattern as the matched string.
If the pattern is matched without the :sigspace modifier, case info is carried across on a character by character basis. If the right string is longer than the left one, the case of the final character is replicated. Titlecase is carried across if possible regardless of whether the resulting letter is at the beginning of a word or not; if there is no titlecase character available, the corresponding uppercase character is used. (This policy can be modified within a lexical scope by a language-dependent Unicode declaration to substitute titlecase according to the orthographic rules of the specified language.) Characters that carry no case information leave their corresponding replacement character unchanged.
If the pattern is matched with :sigspace, then a slightly smarter algorithm is used which attempts to determine if there is a uniform capitalization policy over each matched word, and applies the same policy to each replacement word. If there doesn't seem to be a uniform policy on the left, the policy for each word is carried over word by word, with the last pattern word replicated if necessary. If a word does not appear to have a recognizable policy, the replacement word is translated character for character as in the non-sigspace case. Recognized policies include:
lc()
uc()
ucfirst(lc())
lcfirst(uc())
capitalize()
In any case, only the officially matched string part of the pattern match counts, so any sort of lookahead or contextual matching is not included in the analysis.
:a (or :ignoreaccent) modifier scopes exactly like :ignorecase except that it ignores accents instead of case. It is equivalent to taking each grapheme (in both target and pattern), converting both to NFD (maximally decomposed) and then comparing the two base characters (Unicode non-mark characters) while ignoring any trailing mark characters. The mark characters are ignored only for the purpose of determining the truth of the assertion; the actual text matched includes all ignored characters, including any that follow the final base character.
The :aa (or :sameaccent) variant may be used on a substitution to change the substituted string to the same accent pattern as the matched string. Accent info is carried across on a character by character basis. If the right string is longer than the left one, the remaining characters are substituted without any modification. (Note that NFD/NFC distinctions are usually immaterial, since Perl encapsulates that in grapheme mode.) Under :sigspace the preceding rules are applied word by word.
:c (or :continue) modifier causes the pattern to continue scanning from the specified position (defaulting to $/.to):
m:c($p)/ pattern / # start scanning at position $p
Note that this does not automatically anchor the pattern to the starting location. (Use :p for that.) The pattern you supply to split has an implicit :c modifier.
String positions are of type StrPos and should generally be treated as opaque.
:p (or :pos) modifier causes the pattern to try to match only at the specified string position:
m:pos($p)/ pattern / # match at position $p
If the argument is omitted, it defaults to $/.to. (Unlike in Perl 5, the string itself has no clue where its last match ended.) All subrule matches are implicitly passed their starting position. Likewise, the pattern you supply to a Perl macro's is parsed trait has an implicit :p modifier.
Note that
m:c($p)/pattern/
is roughly equivalent to
m:p($p)/.*? <( pattern )> /
:s (:sigspace) modifier causes whitespace sequences to be considered "significant"; they are replaced by a whitespace matching rule, <.ws>. That is,
m:s/ next cmd '=' <condition>/
is the same as:
m/ <.ws> next <.ws> cmd <.ws> '=' <.ws> <condition>/
which is effectively the same as:
m/ \s* next \s+ cmd \s* '=' \s* <condition>/
But in the case of
m:s{(a|\*) (b|\+)}
or equivalently,
m { (a|\*) <.ws> (b|\+) }
<.ws> can't decide what to do until it sees the data. It still does the right thing. If not, define your own ws and :sigspace will use that.
In general you don't need to use :sigspace within grammars because the parser rules automatically handle whitespace policy for you. In this context, whitespace often includes comments, depending on how the grammar chooses to define its whitespace rule. Although the default <.ws> subrule recognizes no comment construct, any grammar is free to override the rule. The <.ws> rule is not intended to mean the same thing everywhere.
It's also possible to pass an argument to :sigspace specifying a completely different subrule to apply. This can be any rule, it doesn't have to match whitespace. When discussing this modifier, it is important to distinguish the significant whitespace in the pattern from the "whitespace" being matched, so we'll call the pattern's whitespace sigspace, and generally reserve whitespace to indicate whatever <.ws> matches in the current grammar. The correspondence between sigspace and whitespace is primarily metaphorical, which is why the correspondence is both useful and (potentially) confusing.
The :ss (or :samespace) variant may be used on substitutions to do smart space mapping. For each sigspace-induced call to <ws> on the left, the matched whitespace is copied over to the corresponding slot on the right, as represented by a single whitespace character in the replacement string wherever space replacement is desired. If there are more whitespace slots on the right than the left, those righthand characters remain themselves. If there are not enough whitespace slots on the right to map all the available whitespace slots from the match, the algorithm tries to minimize information loss by randomly splicing "common" whitespace characters out of the list of whitespace. From least valuable to most, the pecking order is:
spaces
tabs
all other horizontal whitespace, including Unicode
newlines (including crlf as a unit)
all other vertical whitespace, including Unicode
The primary intent of these rules is to minimize format disruption when substitution happens across line boundaries and such. There is, of course, no guarantee that the result will be exactly what a human would do.
The :s modifier is considered sufficiently important that match variants are defined for them:
mm/match some words/ # same as m:sigspace
ss/match some words/replace those words/ # same as s:samespace
Note that ss/// is defined in terms of :ss, so:
$_ = "a b\nc\td";
ss/b c d/x y z/;
ends up with a value of "a x\ny\tz".
m:bytes / .**2 / # match two bytes
m:codes / .**2 / # match two codepoints
m:graphs / .**2 / # match two language-independent graphemes
m:chars / .**2 / # match two characters at current max level
There are corresponding pragmas to default to these levels. Note that the :chars modifier is always redundant because dot always matches characters at the highest level allowed in scope. This highest level may be identical to one of the other three levels, or it may be more specific than :graphs when a particular language's character rules are in use. Note that you may not specify language-dependent character processing without specifying which language you're depending on. [Conjecture: the :chars modifier could take an argument specifying which language's rules to use for this match.]
:Perl5/:P5 modifier allows Perl 5 regex syntax to be used instead. (It does not go so far as to allow you to put your modifiers at the end.) For instance,
m:P5/(?mi)^(?:[a-z]|\d){1,2}(?=\s)/
is equivalent to the Perl 6 syntax:
m/ :i ^^ [ <[a..z]> || \d ] ** 1..2 <?before \s> /
x, it means repetition. Use :x(4) for the general form. So
s:4x [ (<.ident>) '=' (\N+) $$] = "$0 => $1";
is the same as:
s:x(4) [ (<.ident>) '=' (\N+) $$] = "$0 => $1";
which is almost the same as:
s:c[ (<.ident>) '=' (\N+) $$] = "$0 => $1" for 1..4;
except that the string is unchanged unless all four matches are found. However, ranges are allowed, so you can say :x(1..4) to change anywhere from one to four matches.
st, nd, rd, or th, it means find the Nth occurrence. Use :nth(3) for the general form. So
s:3rd/(\d+)/@data[$0]/;
is the same as
s:nth(3)/(\d+)/@data[$0]/;
which is the same as:
m/(\d+)/ && m:c/(\d+)/ && s:c/(\d+)/@data[$0]/;
Lists and junctions are allowed: :nth(1|2|3|5|8|13|21|34|55|89).
So are closures: :nth({.is_fibonacci})
:ov (:overlap) modifier, the current regex will match at all possible character positions (including overlapping) and return all matches in list context, or a disjunction of matches in item context. The first match at any position is returned. The matches are guaranteed to be returned in left-to-right order with respect to the starting positions.
$str = "abracadabra";
if $str ~~ m:overlap/ a (.*) a / {
@substrings = @@(); # bracadabr cadabr dabr br
}
:ex (:exhaustive) modifier, the current regex will match every possible way (including overlapping) and return all matches in a list context, or a disjunction of matches in item context. The matches are guaranteed to be returned in left-to-right order with respect to the starting positions. The order within each starting position is not guaranteed and may depend on the nature of both the pattern and the matching engine. (Conjecture: or we could enforce backtracking engine semantics. Or we could guarantee no order at all unless the pattern starts with "::" or some such to suppress DFAish solutions.)
$str = "abracadabra";
if $str ~~ m:exhaustive/ a (.*?) a / {
say "@()"; # br brac bracad bracadabr c cad cadabr d dabr br
}
Note that the ~~ above can return as soon as the first match is found, and the rest of the matches may be performed lazily by @().
:rw modifier causes this regex to claim the current string for modification rather than assuming copy-on-write semantics. All the captures in $/ become lvalues into the string, such that if you modify, say, $1, the original string is modified in that location, and the positions of all the other fields modified accordingly (whatever that means). In the absence of this modifier (especially if it isn't implemented yet, or is never implemented), all pieces of $/ are considered copy-on-write, if not read-only.
[Conjecture: this should really associate a pattern with a string variable, not a (presumably immutable) string value.]
:keepall modifier causes this regex and all invoked subrules to remember everything, even if the rules themselves don't ask for their subrules to be remembered. This is for forcing a grammar that throws away whitespace and comments to keep them instead.:ratchet modifier causes this regex to not backtrack by default. (Generally you do not use this modifier directly, since it's implied by token and rule declarations.) The effect of this modifier is to imply a : after every construct that could backtrack, including bare *, +, and ? quantifiers, as well as alternations. (Note: for portions of patterns subject to longest-token analysis, a : is ignored in any case, since there will be no backtracking necessary.):panic modifier causes this regex and all invoked subrules to try to backtrack on any rules that would otherwise default to not backtracking because they have :ratchet set. Never panic unless you're desperate and want the pattern matcher to do a lot of unnecessary work. If you have an error in your grammar, it's almost certainly a bad idea to fix it by backtracking.:i, :s, :Perl5, and Unicode-level modifiers can be placed inside the regex (and are lexically scoped):
m/:s alignment '=' [:i left|right|cent[er|re]] /
As with modifiers outside, only parentheses are recognized as valid brackets for args to the adverb. In particular:
m/:foo[xxx]/ Parses as :foo [xxx]
m/:foo{xxx}/ Parses as :foo {xxx}
m/:foo<xxx>/ Parses as :foo <xxx>
m:fuzzy/pattern/;
m:fuzzy('bare')/pattern/;
m:fuzzy (pattern);
or you'll end up with:
m:fuzzy(fuzzyargs); pattern ;
. now matches any character including newline. (The /s modifier is gone.)^ and $ now always match the start/end of a string, like the old \A and \z. (The /m modifier is gone.) On the right side of an embedded ~~ or !~~ operator they always match the start/end of the indicated submatch because that submatch is logically being treated as a separate string.$ no longer matches an optional preceding \n so it's necessary to say \n?$ if that's what you mean.\n now matches a logical (platform independent) newline not just \x0a.\A, \Z, and \z metacharacters are gone./x is default:
# now always introduces a comment. If followed by an opening bracket character (and if not in the first column), it introduces an embedded comment that terminates with the closing bracket. Otherwise the comment terminates at the newline.:sigspace modifier described above).^^ and $$ match line beginnings and endings. (The /m modifier is gone.) They are both zero-width assertions. $$ matches before any \n (logical newline), and also at the end of the string if the final character was not a \n. ^^ always matches the beginning of the string and after any \n that is not the final character in the string.. matches an anything, while \N matches an anything except newline. (The /s modifier is gone.) In particular, \N matches neither carriage return nor line feed.& metacharacter separates conjunctive terms. The patterns on either side must match with the same beginning and end point. Note: if you don't want your two terms to end at the same point, then you really want to use a lookahead instead.
As with the disjunctions | and ||, conjuctions come in both & and && forms. The & form is considered declarative rather than procedural; it allows the compiler and/or the run-time system to decide which parts to evaluate first, and it is erroneous to assume either order happens consistently. The && form guarantees left-to-right order, and backtracking makes the right argument vary faster than the left. In other words, && and || establish sequence points. The left side may be backtracked into when backtracking is allowed into the construct as a whole.
The & operator is list associative like |, but has slightly tighter precedence. Likewise && has slightly tighter precedence than ||. As with the normal junctional and short-circuit operators, & and | are both tighter than && and ||.
~~ and !~~ operators cause a submatch to be performed on whatever was matched by the variable or atom on the left. String anchors consider that submatch to be the entire string. So, for instance, you can ask to match any identifier that does not contain the word "moose":
<ident> !~~ 'moose'
In contrast
<ident> !~~ ^ 'moose' $
would allow any identifier (including any identifier containing "moose" as a substring) as long as the identifier as a whole is not equal to "moose". (Note the anchors, which attach the submatch to the beginning and end of the identifier as if that were the entire match.) When used as part of a longer match, for clarity it might be good to use extra brackets:
[ <ident> !~~ ^ 'moose' $ ]
The precedence of ~~ and !~~ fits in between the junctional and sequential versions of the logical operators just as it does in normal Perl expressions (see S03). Hence
<ident> !~~ 'moose' | 'squirrel'
parses as
<ident> !~~ [ 'moose' | 'squirrel' ]
while
<ident> !~~ 'moose' || 'squirrel'
parses as
[ <ident> !~~ 'moose' ] || 'squirrel'
~ operator is a helper for matching nested subrules with a specific terminator as the goal. It appears to be placed between the opening and closing bracket, like so:
'(' ~ ')' <expression>
However, it mostly ignores the left argument, and operates on the next two atoms (which may be quantified). Its operation on those next two atoms is to "twiddle" them so that they are actually matched in reverse order. Hence the expression above, at first blush, is merely shortand for:
'(' <expression> ')'
But beyond that, when it rewrites the atoms it also inserts the apparatus that will set up the inner expression to recognize the terminator, and to produce an appropriate error message if the inner expression does not terminate on the required closing atom. So it really does pay attention to the left bracket as well, and it actually rewrites our example to something more like:
$<OPEN> = '(' <SETGOAL: ')'> <expression> [ $GOAL || <FAILGOAL> ]
Note that you can use this construct to set up expectations for a closing construct even when there's no opening bracket:
<null> ~ ')' \d+
By default the error message uses the name of the current rule as an indicator of the abstract goal of the parser at that point. However, often this is not terribly informative, especially when rules are named according to an internal scheme that will not make sense to the user. The :dba("doing business as") adverb may be used to set up a more informative name for what the following code is trying to parse:
token postfix:sym<[ ]> {
:dba('array subscript')
'[' ~ ']' <expression>
}
Then instead of getting a message like:
Unable to parse expression in postfix:sym<[ ]>; couldn't find final ']'
you'll get a message like:
Unable to parse expression in array subscript; couldn't find final ']'
(The :dba adverb may also be used to give names to alternations and alternatives, which helps the lexer give better error messages.)
(...) still delimits a capturing group. However the ordering of these groups is hierarchical rather than linear. See "Nested subpattern captures".[...] is no longer a character class. It now delimits a non-capturing group.{...} is no longer a repetition quantifier. It now delimits an embedded closure. It is always considered procedural rather than declarative; it establishes a sequence point between what comes before and what comes after. (To avoid this use the <?{...}> assertion syntax instead.) / (\S+) { print "string not blank\n"; $text = $0; }
\s+ { print "but does contain whitespace\n" }
/
An explicit reduction using the make function sets the result object for this match:
/ (\d) { make $0.sqrt } Remainder /;
This has the effect of capturing the square root of the numified string, instead of the string. The Remainder part is matched but is not returned unless the first make is later overridden by another make.
These closures are invoked with a topic ($_) of the current match state (a Cursor object). Within a closure, the instantaneous position within the search is denoted by the .pos method on that object. As with all string positions, you must not treat it as a number unless you are very careful about which units you are dealing with.
The Cursor object can also return the original item that we are matching against; this is available from the .orig method.
The closure is also guaranteed to start with a $/ Match object representing the match so far. However, if the closure does its own internal matching, its $/ variable will be rebound to the result of that match until the end of the embedded closure. (The match will actually continue with the current value of the $¢ object after the closure. $/ and $¢ just start out the same in your closure.)
fail:
/ (\d+) { $0 < 256 or fail } /
Since closures establish a sequence point, they are guaranteed to be called at the canonical time even if the optimizer could prove that something after them can't match. (Anything before is fair game, however. In particular, a closure often serves as the terminator of a longest-token pattern.)
** for maximal matching, with a corresponding **? for minimal matching. (All such quantifier modifiers now go directly after the **.) Space is allowed on either side of the complete quantifier. This space is considered significant under :sigspace, and will be distributed as a call to <.ws> between all the elements of the match but not on either end.
The next token will determine what kind of repetition is desired:
If the next thing is an integer, then it is parsed as either as an exact count or a range:
. ** 42 # match exactly 42 times
<item> ** 3..* # match 3 or more times
This form is considered declarational.
If you supply a closure, it should return either an Int or a Range object.
'x' ** {$m} # exact count returned from closure
<foo> ** {$m..$n} # range returned from closure
/ value was (\d **? {1..6}) with ([ <alpha>\w* ]**{$m..$n}) /
It is illegal to return a list, so this easy mistake fails:
/ [foo] ** {1,3} /
The closure form is always considered procedural, so the item it is modifying is never considered part of the longest token.
If you supply any other atom (which may be quantified), it is interpreted as a separator (such as an infix operator), and the initial item is quantified by the number of times the separator is seen between items:
<alt> ** '|' # repetition controlled by presence of character
<addend> ** <addop> # repetition controlled by presence of subrule
<item> ** [ \!?'==' ] # repetition controlled by presence of operator
<file>**\h+ # repetition controlled by presence of whitespace
A successful match of such a quantifier always ends "in the middle", that is, after the initial item but before the next separator. Therefore
/ <ident> ** ',' /
can match
foo
foo,bar
foo,bar,baz
but never
foo,
foo,bar,
It is legal for the separator to be zero-width as long as the pattern on the left progresses on each iteration:
. ** <?same> # match sequence of identical characters
The separator never matches independently of the next item; if the separator matches but the next item fails, it backtracks all the way back through the separator. Likewise, this matching of the separator does not count as "progress" under :ratchet semantics unless the next item succeeds.
When significant space is used under :sigspace with the separator form, it applies on both sides of the separator, so
mm/<element> ** ','/
mm/<element>** ','/
mm/<element> **','/
all allow whitespace around the separator like this:
/ <element>[<.ws>','<.ws><element>]* /
while
mm/<element>**','/
excludes all significant whitespace:
/ <element>[','<element>]* /
Of course, you can always match whitespace explicitly if necessary, so to allow whitespace after the comma but not before, you can say:
/ <element>**[','\s*] /
<...> are now extensible metasyntax delimiters or assertions (i.e. they replace Perl 5's crufty (?...) syntax).'...' literal (i.e. it does not treat the interpolated string as a subpattern). In other words, a Perl 6:
/ $var /
is like a Perl 5:
/ \Q$var\E /
However, if $var contains a Regex object, instead of attempting to convert it to a string, it is called as a subrule, as if you said <$var>. (See assertions below.) This form does not capture, and it fails if $var is tainted.
However, a variable used as the left side of an alias or submatch operator is not used for matching.
$x = <ident>
$0 ~~ <ident>
If you do want to match $0 again and then use that as the submatch, you can force the match using double quotes:
"$0" ~~ <ident>
On the other hand, it is non-sensical to alias to something that is not a variable:
"$0" = <ident> # ERROR
$0 = <ident> # okay
$x = <ident> # okay, temporary capture
$<x> = <ident> # okay, persistent capture
<x=ident> # same thing
Variables declared in capture aliases are lexically scoped to the rest of the regex. You should not confuse this use of = with either ordinary assignment or ordinary binding. You should read the = more like the pseudoassignment of a declarator than like normal assignment. It's more like the ordinary := operator, since at the level regexes work, strings are immutable, so captures are really just precomputed substr values. Nevertheless, when you eventually use the values independently, the substr may be copied, and then it's more like it was an assignment originally.
Capture variables of the form $<ident> may persist beyond the lexical scope; if the match succeeds they are remembered in the Match object's hash, with a key corresponding to the variable name's identifier. Likewise bound numeric variables persist as $0, etc.
The capture performed by = creates a new lexical variable if it does not already exist in the current lexical scope. To capture to an outer lexical variable you must supply an OUTER:: as part of the name, or perform the assignment from within a closure.
$x = [...] # capture to our own lexical $x
$OUTER::x = [...] # capture to existing lexical $x
[...] -> $tmp { let $x = $tmp } # capture to existing lexical $x
Note however that let (and temp) are not guaranteed to be thread safe on shared variables, so don't do that.
/ @cmds /
is matched as if it were an alternation of its elements. Ordinarily it matches using junctive semantics:
/ [ @cmds[0] | @cmds[1] | @cmds[2] | ... ] /
However, if it is a direct member of a || list, it uses sequential matching semantics, even it's the only member of the list. Conveniently, you can put || before the first member of an alternation, hence
/ || @cmds /
is equivalent to
/ [ @cmds[0] || @cmds[1] || @cmds[2] || ... ] /
Or course, you can also
/ | @cmds /
to be clear that you mean junctive semantics.
As with a scalar variable, each element is matched as a literal unless it happens to be a Regex object, in which case it is matched as a subrule. As with scalar subrules, a tainted subrule always fails. All string values pay attention to the current :ignorecase and :ignoreaccent settings, while Regex values use their own :ignorecase and :ignoreaccent settings.
When you get tired of writing:
token sigil { '$' | '@' | '@@' | '%' | '&' | '::' }
you can write:
token sigil { < $ @ @@ % & :: > }
as long as you're careful to put a space after the initial angle so that it won't be interpreted as a subrule. With the space it is parsed like angle quotes in ordinary Perl 6 and treated as a literal array value.
<sym>, like this:
proto token sigil { }
multi token sigil:sym<$> { <sym> }
multi token sigil:sym<@> { <sym> }
multi token sigil:sym<@@> { <sym> }
multi token sigil:sym<%> { <sym> }
multi token sigil:sym<&> { <sym> }
multi token sigil:sym<::> { <sym> }
(The multi is optional and generally omitted with a grammar.)
This can be viewed as a form of multiple dispatch, except that it's based on longest-token matching rather than signature matching. The advantage of writing it this way is that it's easy to add additional rules to the same category in a derived grammar. All of them will be matched in parallel when you try to match /<sigil>/.
If there are formal parameters on multi regex methods, matching still proceeds via longest-token rules first. If that results in a tie, a normal multiple dispatch is made using the arguments to the remaining variants, assuming they can be differentiated by type.
In a context requiring a set of initial token patterns, the initial token patterns are taken to be each key plus any initial token pattern matched by the corresponding value (if the value is a string or regex). The token patterns are considered to be canonicalized in the same way as any surrounding context, so for instance within a case-insensitive context the hash keys must match insensitively also.
Subsequent matching depends on the hash value:
"", nothing special happens except that the key match succeeds.Regex object, it is executed as a subrule, with an initial position after the matched key. (This is further described below under the <%hash> notation.) As with scalar subrules, a tainted subrule always fails, and no capture is attempted.All hash keys, and values that are strings, pay attention to the :ignorecase and :ignoreaccent settings. (Subrules maintain their own case settings.)
You may combine multiple hashes under the same longest-token consideration by using declarative alternation:
%statement | %prefix | %term
This means that, despite being in a later hash, %term<food> will be selected in preference to %prefix<foo> because it's the longer token. However, if there is a tie, the earlier hash wins, so %statement<if> hides any %prefix<if> or %term<if>.
In contrast, if you use a procedural alternation:
[ %prefix || %term ]
a %prefix<foo> would be selected in preference to a %term<food>. (Which is not what you usually want if your language is to do longest-token consistently.)
<...>)Both < and > are metacharacters, and are usually (but not always) used in matched pairs. (Some combinations of metacharacters function as standalone tokens, and these may include angles. These are described below.) Most assertions are considered declarative; procedural assertions will be marked as exceptions.
For matched pairs, the first character after < determines the nature of the assertion:
< adam & eve > # equivalent to [ 'adam' | '&' | 'eve' ]
/ <sign>? <mantissa> <exponent>? /
The first character after the identifier determines the treatment of the rest of the text before the closing angle. The underlying semantics is that of a function or method call, so if the first character is a left parenthesis, it really is a call:
<foo('bar')>
If the first character after the identifier is an =, then the identifier is taken as an alias for what follows. In particular,
<foo=bar>
is just shorthand for
$<foo> = <bar>
If the first character after the identifier is whitespace, the subsequent text (following any whitespace) is passed as a regex, so:
<foo bar>
is more or less equivalent to
<foo(/bar/)>
To pass a regex with leading whitespace you must use the parenthesized form.
If the first character is a colon followed by whitespace, the rest of the text is taken as a list of arguments to the method, just as in ordinary Perl syntax. So these mean the same thing:
<foo('foo', $bar, 42)>
<foo: 'foo', $bar, 42>
No other characters are allowed after the initial identifier.
Subrule matches are considered declarative to the extent that the front of the subrule is itself considered declarative. If a subrule contains a sequence point, then so does the subrule match. Longest-token matching does not proceed past such a subrule, for instance.
. causes a named assertion not to capture what it matches (see "Subrule captures". For example:
/ <ident> <ws> / # $/<ident> and $/<ws> both captured
/ <.ident> <ws> / # only $/<ws> captured
/ <.ident> <.ws> / # nothing captured
The assertion is otherwise parsed identically to an assertion beginning with an identifier, provided the next thing after the dot is an identifier. As with the identifier form, any extra arguments pertaining to the matching engine are automatically supplied to the argument list.
If the dot is not followed by an identifier, it is parsed as a "dotty" postfix of some type, such as an indirect method call:
<.$indirect($depth, $binding, $fate, @args)>
In this case the object passed as the invocant is the current match state, and the method is expected to return a new match state object. The extra pattern matching arguments ($depth, $binding, and $fate) must be supplied explicitly.
The non-capturing behavior may be overridden with a :keepall.
$ indicates an indirect subrule. The variable must contain either a Regex object, or a string to be compiled as the regex. The string is never matched literally.
Such an assertion is not captured. (No assertion with leading punctuation is captured by default.) You may always capture it explicitly, of course.
A subrule is considered declarative to the extent that the front of it is declarative, and to the extent that the variable doesn't change. Prefix with a sequence point to defeat repeated static optimizations.
:: indicates a symbolic indirect subrule:
/ <::($somename)> /
The variable must contain the name of a subrule. By the rules of single method dispatch this is first searched for in the current grammar and its ancestors. If this search fails an attempt is made to dispatch via MMD, in which case it can find subrules defined as multis rather than methods. This form is not captured by default. It is always considered procedural, not declarative.
@ matches like a bare array except that each element is treated as a subrule (string or Regex object) rather than as a literal. That is, a string is forced to be compiled as a subrule instead of being matched literally. (There is no difference for a Regex object.)
This assertion is not automatically captured.
% matches like a bare hash except that a string value is always treated as a subrule, even if it is a string that must be compiled to a regex at match time. (Numeric values may still indicate "false match". and a closure may do whatever it likes.)
This assertion is not automatically captured.
As with bare hash, the longest key matches according to the venerable longest-token rule. [Conjecture: <%foo> may not be supported in 6.0, or may be retargeted to matching an abbreviation table.]
{ indicates code that produces a regex to be interpolated into the pattern at that point as a subrule:
/ (<.ident>) <{ %cache{$0} //= get_body_for($0) }> /
The closure is guaranteed to be run at the canonical time; it declares a sequence point, and is considered to be procedural.
& interpolates the return value of a subroutine call as a regex. Hence
<&foo()>
is short for
<{ foo() }>
This is considered procedural.
Regex object, it is not recompiled. If it is a string, the compiled form is cached with the string so that it is not recompiled next time you use it unless the string changes. (Any external lexical variable names must be rebound each time though.) Subrules may not be interpolated with unbalanced bracketing. An interpolated subrule keeps its own inner match result as a single item, so its parentheses never count toward the outer regexes groupings. (In other words, parenthesis numbering is always lexically scoped.)?{ or !{ indicates a code assertion:
/ (\d**1..3) <?{ $0 < 256 }> /
/ (\d**1..3) <!{ $0 < 256 }> /
Similar to:
/ (\d**1..3) { $0 < 256 or fail } /
/ (\d**1..3) { $0 < 256 and fail } /
Unlike closures, code assertions are considered declarative; they are not guaranteed to be run at the canonical time if the optimizer can prove something later can't match. So you can sneak in a call to a non-canonical closure that way:
token { foo .* <?{ do { say "Got here!" } or 1 }> .* bar }
The do block is unlikely to run unless the string ends with "bar".
[ indicates an enumerated character class. Ranges in enumerated character classes are indicated with ".." rather than "-".
/ <[a..z_]>* /
Whitespace is ignored within square brackets:
/ <[ a..z _ ]>* /
- indicates a complemented character class:
/ <-[a..z_]> <-alpha> /
/ <- [a..z_]> <- alpha> / # whitespace allowed after -
This is essentially the same as using negative lookahead and dot:
/ <![a..z_]> . <!alpha> . /
Whitespace is ignored after the initial -.
+ may also be supplied to indicate that the following character class is to matched in a positive sense.
/ <+[a..z_]>* /
/ <+[ a..z _ ]>* /
/ <+ [ a .. z _ ] >* / # whitespace allowed after +
/ <[a..z] - [aeiou] + xdigit> / # consonant or hex digit
A named character class may be used by itself:
<alpha>
However, in order to combine classes you must prefix a named character class with + or -.
<.> matches any logical grapheme (including a Unicode combining character sequences):
/ seekto = <.> / # Maybe a combined char
Same as:
/ seekto = [:graphs .] /
! indicates a negated meaning (always a zero-width assertion):
/ <!before _ > / # We aren't before an _
Note that <!alpha> is different from <-alpha>. /<-alpha>/ is a complemented character class equivalent to /<!before <alpha>> ./, whereas <!alpha> is a zero-width assertion equivalent to a /<!before <alpha>>/ assertion.
Note also that as a metacharacter ! doesn't change the parsing rules of whatever follows (unlike, say, + or -).
? indicates a positive zero-width assertion, and like ! merely reparses the rest of the assertion recursively as if the ? were not there. In addition to forcing zero-width, it also suppresses any named capture:
<alpha> # match a letter and capture to $alpha (eventually $<alpha>)
<.alpha> # match a letter, don't capture
<?alpha> # match null before a letter, don't capture
The special named assertions include:
/ <?before pattern> / # lookahead
/ <?after pattern> / # lookbehind
/ <?same> / # true between two identical characters
/ <.ws> / # match "whitespace":
# \s+ if it's between two \w characters,
# \s* otherwise
/ <?at($pos)> / # match only at a particular StrPos
# short for <?{ .pos === $pos }>
# (considered declarative until $pos changes)
It is legal to use any of these assertions as named captures by omitting the punctuation at the front. However, capture entails some overhead in both memory and computation, so in general you want to suppress that for data you aren't interested in preserving.
The after assertion implements lookbehind by reversing the syntax tree and looking for things in the opposite order going to the left. It is illegal to do lookbehind on a pattern that cannot be reversed.
Note: the effect of a forward-scanning lookbehind at the top level can be achieved with:
/ .*? prestuff <( mainpat )> /
<...>, <???>, and <!!!> special tokens have the same "not-defined-yet" meanings within regexes that the bare elipses have in ordinary code. (However, by the recursive rules above, <!!> is equivalent to <?>, which matches the null string. Likewise <!!before x> is just like <?before x>, except that it is ignored by the longest-token matcher.)* indicates that the following pattern allows a partial match. It always succeeds after matching as many characters as possible. (It is not zero-width unless 0 characters match.) For instance, to match a number of abbreviations, you might write any of:
s/ ^ G<*n|enesis> $ /gen/ or
s/ ^ Ex<*odus> $ /ex/ or
s/ ^ L<*v|eviticus> $ /lev/ or
s/ ^ N<*m|umbers> $ /num/ or
s/ ^ D<*t|euteronomy> $ /deut/ or
...
/ (<* <foo bar baz> >) /
/ <short=*@abbrev> / and return %long{$<short>} || $<short>;
The pattern is restricted to declarative forms that can be rewritten as nested optional character matches. Sequence information may not be discarded while making all following characters optional. That is, it is not sufficient to rewrite:
<*xyz>
as:
x? y? z? # bad, would allow xz
Instead, it must be implemented as:
[x [y z?]?]? # allow only x, xy, xyz (and '')
Explicit quantifiers are allowed on single characters, so this:
<* a b+ c | ax*>
is rewritten as something like:
[a [b+]? c?]? | [a x*]?
In the latter example we're assuming the DFA token matcher is going to give us the longest match regardless. It's also possible that quantified multichar sequences can be recursively remapped:
<* 'ab'+> # match a, ab, ababa, etc. (but not aab!)
==> [ 'ab'* <*ab> ]
==> [ 'ab'* [a b?]? ]
[Conjecture: depending on how fancy we get, we might (or might not) be able to autodetect ambiguities in <*@abbrev> and refuse to generate ambiguous abbreviations (although exact match of a shorter abbrev should always be allowed even if it's the prefix of a longer abbreviation). If it is not possible, then the user will have to check for ambiguities after the match. Note also that the array form is assuming the array doesn't change often. If it does, the longest-token matcher has to be recalculated, which could get expensive.]
~~ indicates a recursive call back into some or all of the current rule. An optional argument indicates which subpattern to re-use, and if provided must resolve to a single subpattern. If omitted, the entire pattern is called recursively:
<~~> # call myself recursively
<~~0> # match according to $0's pattern
<~~foo> # match according to $foo's pattern
Note that this rematches the pattern associated with the name, not the string matched. So
$_ = "foodbard"
/ ( foo | bar ) d $0 / # fails; doesn't match "foo" literally
/ ( foo | bar ) d <$0> / # fails; doesn't match /foo/ as subrule
/ ( foo | bar ) d <~~0> / # matches using rule associated with $0
The last is equivalent to
/ ( foo | bar ) d ( foo | bar ) /
Note that the "self" call of
/ <term> <operator> <~~> /
calls back into this anonymous rule as a subrule, and is implicitly anchored to the end of the operator as any other subrule would be. Despite the fact that the outer rule scans the string, the inner call to it does not.
Note that a consequence of previous section is that you also get
<!~~>
for free, which fails if the current rule would match again at this location.
The following tokens include angles but are not required to balance:
<( token indicates the start of a result capture, while the corresponding )> token indicates its endpoint. When matched, these behave as assertions that are always true, but have the side effect of setting the .from and .to attributes of the match object. That is:
/ foo <( \d+ )> bar /
is equivalent to:
/ <?after foo> \d+ <?before bar> /
except that the scan for "foo" can be done in the forward direction, while a lookbehind assertion would presumably scan for \d+ and then match "foo" backwards. The use of <(...)> affects only the meaning of the result object and the positions of the beginning and ending of the match. That is, after the match above, $() contains only the digits matched, and $/.to is pointing to after the digits. Other captures (named or numbered) are unaffected and may be accessed through $/.
These tokens are considered declarative, but may force backtracking behavior.
« or << token indicates a left word boundary. A » or >> token indicates a right word boundary. (As separate tokens, these need not be balanced.) Perl 5's \b is replaced by a <?wb> "word boundary" assertion, while \B becomes <!wb>. (None of these are dependent on the definition of <.ws>, but only on the \w definition of "word" characters.)\p and \P properties become intrinsic grammar rules such as (<alpha> and <-alpha>). They may be combined using the above-mentioned character class notation: <[_]+alpha+digit>. Regardless of the higher-level character class names, low-level Unicode properties are always available with a prefix of is. Hence, <+isLu+isLt> is equivalent to <+upper+title>. If you define your own "is" properties they hide any Unicode properties of the same name.\L...\E, \U...\E, and \Q...\E sequences are gone. In the rare cases that need them you can use <{ lc $regex }> etc.\G sequence is gone. Use :p instead. (Note, however, that it makes no sense to use :p within a pattern, since every internal pattern is implicitly anchored to the current position.) See the at assertion below.\1, \2, etc.) are gone; $0, $1, etc. can be used instead, because variables are no longer interpolated.
Numeric variables are assumed to change every time and therefore are considered procedural, unlike normal variables.
\h and \v, match horizontal and vertical whitespace respectively, including Unicode.\s now matches any Unicode whitespace character.\N matches anything except a logical newline; it is the negation of \n.\H matches anything but horizontal whitespace.\V matches anything but vertical whitespace.\T matches anything but a tab.\R matches anything but a return.\F matches anything but a formfeed.\E matches anything but an escape.\X... matches anything but the specified character (specified in hexadecimal).qr/pattern/ regex constructor is gone. regex { pattern } # always takes {...} as delimiters
rx / pattern / # can take (almost any) chars as delimiters
You may not use whitespace or alphanumerics for delimiters. Space is optional unless needed to distinguish from modifier arguments or function parens. So you may use parens as your rx delimiters, but only if you interpose whitespace:
rx ( pattern ) # okay
rx( 1,2,3 ) # tries to call rx function
(This is true for all quotelike constructs in Perl 6.)
$regex = regex :g:s:i { my name is (.*) };
$regex = rx:g:s:i / my name is (.*) /; # same thing
Space is necessary after the final modifier if you use any bracketing character for the delimiter. (Otherwise it would be taken as an argument to the modifier.)
$regex = rx :g :s :i / my name is (.*) /;
qr because it's no longer an interpolating quote-like operator. rx is short for regex, (not to be confused with regular expressions, except when they are).sub {...} constructor. In fact, that analogy runs very deep in Perl 6.{...} is now always a closure (which may still execute immediately in certain contexts and be passed as an object in others), so too a raw /.../ is now always a Regex object (which may still match immediately in certain contexts and be passed as an object in others)./.../ matches immediately in a value context (void, Boolean, string, or numeric), or when it is an explicit argument of a ~~. Otherwise it's a Regex constructor identical to the explicit regex form. So this:
$var = /pattern/;
no longer does the match and sets $var to the result. Instead it assigns a Regex object to $var.
m{...} or rx{...}:
$match = m{pattern}; # Match regex immediately, assign result
$regex = rx{pattern}; # Assign regex expression itself
@list = split /pattern/, $str;
are now just consequences of the normal semantics.
grep:
sub my_grep($selector, *@list) {
given $selector {
when Regex { ... }
when Code { ... }
when Hash { ... }
# etc.
}
}
When you call my_grep, the first argument is bound in item context, so passing {...} or /.../ produces a Code or Regex object, which the switch statement then selects upon. (Normal grep just lets a smartmatch operator do all the work.)
rx has variants, so does the regex declarator. In particular, there are two special variants for use in grammars: token and rule.
A token declaration:
token ident { [ <alpha> | _ ] \w* }
never backtracks by default. That is, it likes to commit to whatever it has scanned so far. The above is equivalent to
regex ident { [ <alpha>: | _: ]: \w*: }
but rather easier to read. The bare *, +, and ? quantifiers never backtrack in a token unless some outer regex has specified a :panic option that applies. If you want to prevent even that, use *:, +:, or ?: to prevent any backtracking into the quantifier. If you want to explicitly backtrack, append either a ? or a ! to the quantifier. The ? forces minimal matching as usual, while the ! forces greedy matching. The token declarator is really just short for
regex :ratchet { ... }
The other is the rule declarator, for declaring non-terminal productions in a grammar. Like a token, it also does not backtrack by default. In addition, a rule regex also assumes :sigspace. A rule is really short for:
regex :ratchet :sigspace { ... }
?...? syntax (succeed once) was rarely used and can be now emulated more cleanly with a state variable:
$result = do { state $x ||= m/ pattern /; } # only matches first time
To reset the pattern, simply say $x = 0. Though if you want $x visible you'd have to avoid using a block:
$result = state $x ||= m/ pattern /;
...
$x = 0;
Within those portions of a pattern that are considered procedural rather than declarative, you may control the backtracking behavior.
rx, m, s, and the like. It's also greedy in ordinary regex declarations. In rule and token declarations, backtracking must be explicit.:? or ? to the atom. If the preceding token is a quantifier, the : may be omitted, so *? works just as in Perl 5.:! to the atom. If the preceding token is a quantifier, the : may be omitted. (Perl 5 has no corresponding construct because backtracking always defaults to greedy in Perl 5.): without a subsequent ? or !. Backtracking over a single colon causes the regex engine not to retry the preceding atom:
mm/ \( <expr> [ , <expr> ]*: \) /
(i.e. there's no point trying fewer <expr> matches, if there's no closing parenthesis on the horizon)
To force all the atoms in an expression not to backtrack by default, use :ratchet or rule or token.
mm/ [ if :: <expr> <block>
| for :: <list> <block>
| loop :: <loop_controls>? <block>
]
/
(i.e. there's no point trying to match a different keyword if one was already found but failed). Note that you can still back into such an alternation, so you may also need to put : after it if you also want to disable that. If an explicit or implicit :ratchet has disabled backtracking by supplying an implicit :, you need to put an explicit ! after the alternation to enable backing into another alternative if the first pick fails.
The :: also has the effect of hiding any constant string on the right from "longest token" processing by |. Only the left side is evaluated for initial constancy.
regex ident {
( [<alpha>|_] \w* ) ::: { fail if %reserved{$0} }
|| " [<alpha>|_] \w* "
}
mm/ get <ident>? /
(i.e. using an unquoted reserved word as an identifier is not permitted)
<commit> assertion causes the entire match to fail outright, no matter how many subrules down it happens:
regex subname {
([<alpha>|_] \w*) <commit> { fail if %reserved{$0} }
}
mm/ sub <subname>? <block> /
(i.e. using a reserved word as a subroutine name is instantly fatal to the surrounding match as well)
If commit is given an argument, it's the name of a calling rule that should be committed:
<commit('infix')>
<cut> assertion always matches successfully, and has the side effect of logically deleting the parts of the string already matched. Whether this actually frees up the memory immediately may depend on various interactions among your backreferences, the string implementation, and the garbage collector. In any case, the string will report that it has been chopped off on the front. It's illegal to use <cut> on a string that you do not have write access to.
Attempting to backtrack past a <cut> causes the complete match to fail (like backtracking past a <commit>). This is because there's now no preceding text to backtrack into. This is useful for throwing away successfully processed input when matching from an input stream or an iterator of arbitrary length.
sub and regex extends much further. token ident { [<alpha>|_] \w* }
# and later...
@ids = grep /<ident>/, @strings;
regex serial_number { <[A..Z]> \d**8 }
token type { alpha | beta | production | deprecated | legacy }
in other regexes as named assertions:
rule identification { [soft|hard]ware <type> <serial_number> }
These keyword-declared regexes are officially of type Method, which is derived from Routine.
In general, the anchoring of any subrule call is controlled by context. When a regex, token, or rule method is called as a subrule, the front is anchored to the current position (as with :p), while the end is not anchored, since the calling context will likely wish to continue parsing. However, when such a method is smartmatched directly, it is automatically anchored on both ends to the beginning and end of the string. Thus, you can do direct pattern matching by using an anonymous regex routine as a standalone pattern:
$string ~~ regex { \d+ }
$string ~~ token { \d+ }
$string ~~ rule { \d+ }
and these are equivalent to
$string ~~ m/^ \d+ $/;
$string ~~ m/^ \d+: $/;
$string ~~ m/^ <.ws> \d+: <.ws> $/;
The basic rule of thumb is that the keyword-defined methods never do implicit .*?-like scanning, while the m// and s// quotelike forms do such scanning in the absence of explicit anchoring.
The rx// and // forms can go either way: they scan when used directly within a smartmatch or boolean context, but when called indirectly as a subrule they do not scan. That is, the object returned by rx// behaves like m// when used directly, but like regex {} when used as a subrule:
$pattern = rx/foo/;
$string ~~ $pattern; # equivalent to m/foo/;
$string ~~ /'[' <$pattern> ']'/ # equivalent to /'[foo]'/
/ <prior> /
/ '' /;
/ <?> /;
For example:
split /''/, $string
splits between characters. But then, so does this:
split '', $string
/a|b|c|<?>/
/a|b|c|''/
This makes it easier to catch errors like this:
/a|b|c|/
As a special case, however, the first null alternative in a match like
mm/ [
| if :: <expr> <block>
| for :: <list> <block>
| loop :: <loop_controls>? <block>
]
/
is simply ignored. Only the first alternative is special that way. If you write:
mm/ [
if :: <expr> <block> |
for :: <list> <block> |
loop :: <loop_controls>? <block> |
]
/
it's still an error.
$something = "";
/a|b|c|$something/;
In particular, <?> always matches the null string successfully, and <!> always fails to match anything.
Instead of representing temporal alternation, | now represents logical alternation with declarative longest-token semantics. (You may now use || to indicate the old temporal alternation. That is, | and || now work within regex syntax much the same as they do outside of regex syntax, where they represent junctional and short-circuit OR. This includes the fact that | has tighter precedence than ||.)
Historically regex processing has proceeded in Perl via a backtracking NFA algorithm. This is quite powerful, but many parsers work more efficiently by processing rules in parallel rather than one after another, at least up to a point. If you look at something like a yacc grammar, you find a lot of pattern/action declarations where the patterns are considered in parallel, and eventually the grammar decides which action to fire off. While the default Perl view of parsing is essentially top-down (perhaps with a bottom-up "middle layer" to handle operator precedence), it is extremely useful for user understanding if at least the token processing proceeds deterministically. So for regex matching purposes we define token patterns as those patterns containing no whitespace that can be matched without side effects or self-reference. Basically, Perl automatically derives a lexer from the grammar without you having to write one yourself.
To that end, every regex in Perl 6 is required to be able to distinguish its "pure" patterns from its actions, and return its list of initial token patterns (transitively including the token patterns of any subrule called by the "pure" part of that regex, but not including any subrule more than once, since that would involve self reference, which is not allowed in traditional regular expressions). A logical alternation using | then takes two or more of these lists and dispatches to the alternative that matches the longest token prefix. This may or may not be the alternative that comes first lexically.
However, if two alternatives match at the same length, the tie is broken by one of two methods. If the alternatives are in different grammars, standard MRO (method resolution order) determines which one to try first. If the alternatives are in the same grammar, the textually earlier alternative takes precedence. (If a grammar's rules are defined in more than one file, the results are undefined.)
This longest token prefix corresponds roughly to the notion of "token" in other parsing systems that use a lexer, but in the case of Perl this is largely an epiphenomenon derived automatically from the grammar definition. However, despite being automatically calculated, the set of tokens can be modified by the user; various constructs within a regex declaratively tell the grammar engine that it is finished with the pattern part and starting in on the side effects, so by inserting such constructs the user controls what is considered a token and what is not. The constructs deemed to terminate a token declaration and start the "action" part of the pattern include:
? modifier).{...} action, but not an assertion containing a closure. The closure form of the general **{...} quantifier terminates the longest token, but not the closureless forms.|| or &&.<ws> rule defines whitespace using ||, the longest token is also terminated by any part of the regex or rule that might match whitespace using that rule, including whitespace implicitly matched via :sigspace. (However, token declarations are specifically allowed to recognize whitespace within a token by using such lower-level primitives as \h+ or other character classes.)Subpatterns (captures) specifically do not terminate the token pattern, but may require a reparse of the token to find the location of the subpatterns. Likewise assertions may need to be checked out after the longest token is determined. (Alternately, if DFA semantics are simulated in any of various ways, such as by Thompson NFA, it may be possible to know when to fire off the assertions without backchecks.)
Greedy quantifiers and character classes do not terminate a token pattern. Zero-width assertions such as word boundaries are also okay.
Because such assertions can be part of the token, the lexer engine must be able to recover from the failure of such an assertion and backtrack to the next best token candidate, which might be the same length or shorter, but can never be longer than the current candidate.
For a pattern that starts with a positive lookahead assertion, the assertion is assumed to be more specific than the subsequent pattern, so the lookahead's pattern is treated as the longest token; the longest-token matcher will be smart enough to rematch any text traversed by the lookahead when (and if) it continues the match.
Oddly enough, the token keyword specifically does not determine the scope of a token, except insofar as a token pattern usually doesn't do much matching of whitespace. In contrast, the rule keyword (which assumes :sigspace) defines a pattern that tends to disqualify itself on the first whitespace. So most of the token patterns will end up coming from token declarations. For instance, a token declaration such as
token list_composer { \[ <expr> \] }
considers its "longest token" to be just the left square bracket, because the first thing the expr rule will do is traverse optional whitespace.
The initial token matcher must take into account case sensitivity (or any other canonicalization primitives) and do the right thing even when propagated up to rules that don't have the same canonicalization. That is, they must continue to represent the set of matches that the lower rule would match.
The || form has the old short-circuit semantics, and will not attempt to match its right side unless all possibilities (including all | possibilities) are exhausted on its left. The first || in a regex makes the token patterns on its left available to the outer longest-token matcher, but hides any subsequent tests from longest-token matching. Every || establishes a new longest-token matcher. That is, if you use | on the right side of ||, that right side establishes a new top level scope for longest-token processing for this subexpression and any called subrules. The right side's longest-token automaton is invisible to the left of the || or outside the regex containing the ||.
$/, which is a contextual lexical declared in the outer subroutine that is calling the regex. (A regex declares its own lexical $/ variable, which always refers to the most recent submatch within the rule, if any.) The current match state is kept in the regex's $¢ variable which will eventually get processed into the user's $/ variable when the match completes. if /pattern/ {...}
# or:
/pattern/; if $/ {...}
With :global or :overlap or :exhaustive the boolean is allowed to return true on the first match. The Match object can produce the rest of the results lazily if evaluated in list context.
print %hash{ "{$text ~~ /<.ident>/}" };
# or equivalently:
$text ~~ /<.ident>/ && print %hash{~$/};
But generally you should say ~$/ if you mean ~$/.
$sum += /\d+/;
# or equivalently:
/\d+/; $sum = $sum + $/;
Match object evaluates to its underlying result object. Usually this is just the entire match string, but you can override that by calling make inside a regex:
my $moose = $(m:{
<antler> <body>
{ make Moose.new( body => $<body>.attach($<antler>) ) }
# match succeeds -- ignore the rest of the regex
});
$() is a shorthand for $($/). The result object may be of any type, not just a string.
You may also capture a subset of the match as the result object using the <(...)> construct:
"foo123bar" ~~ / foo <( \d+ )> bar /
say $(); # says 123
In this case the result object is always a string when doing string matching, and a list of one or more elements when doing array matching.
Additionally, the Match object delegates its coerce calls (such as +$match and ~$match) to its underlying result object. The only exception is that Match handles boolean coercion itself, which returns whether the match had succeeded at least once.
This means that these two work the same:
/ <moose> { make $moose as Moose } /
/ <moose> { make $$moose as Moose } /
Match object pretends to be an array of all its positional captures. Hence
($key, $val) = mm/ (\S+) => (\S+)/;
can also be written:
$result = mm/ (\S+) '=>' (\S+)/;
($key, $val) = @$result;
To get a single capture into a string, use a subscript:
$mystring = "{ mm/ (\S+) '=>' (\S+)/[0] }";
To get all the captures into a string, use a zen slice:
$mystring = "{ mm/ (\S+) '=>' (\S+)/[] }";
Or cast it into an array:
$mystring = "@( mm/ (\S+) '=>' (\S+)/ )";
Note that, as a scalar variable, $/ doesn't automatically flatten in list context. Use @() as a shorthand for @($/) to flatten the positional captures under list context. Note that a Match object is allowed to evaluate its match lazily in list context. Use eager @() to force an eager match.
Match object pretends to be a hash of all its named captures. The keys do not include any sigils, so if you capture to variable @<foo> its real name is $/{'foo'} or $/<foo>. However, you may still refer to it as @<foo> anywhere $/ is visible. (But it is erroneous to use the same name for two different capture datatypes.)
Note that, as a scalar variable, $/ doesn't automatically flatten in list context. Use %() as a shorthand for %($/) to flatten as a hash, or bind it to a variable of the appropriate type. As with @(), it's possible for %() to produce its pairs lazily in list context.
$<0 1 2> is equivalent to $/[0,1,2]. This allows you to write slices of intermixed named and numbered captures.$0, $1, etc. are just aliases into $/[0], $/[1], etc. Hence they will all be undefined if the last match failed (unless they were explicitly bound in a closure without using the let keyword).Match objects have methods that provide additional information about the match. For example:
if m/ def <ident> <codeblock> / {
say "Found sub def from index $/.from.bytes ",
"to index $/.to.bytes";
}
The currently defined methods are
$/.from # the initial match position
$/.to # the final match position
$/.chars # $/.to - $/.from
$/.orig # the original match string
$/.text # substr($/.orig, $/.from, $/.chars)
Within the regex the current match state $¢ also provides
.pos # the current match position
This last value may correspond to either $¢.from or $¢.to depending on whether the match is proceeding in a forward or backward direction (the latter case arising inside an <?after ...> assertion).
Match. That is:
$match_obj = $str ~~ /pattern/;
say "Matched" if $match_obj;
$/ variable of the current surroundings. That is:
$str ~~ /pattern/;
say "Matched" if $/;
$_ variable holds the current regex's incomplete Match object, known as a match state. Generally this should not be modified unless you know how to create and propagate match states. All regexes actually return match states even when you think they're returning something else, because the match states keep track of the success and failures of the pattern for you.
Fortunately, when you just want to return a different result object instead of the default Match object, you may associate your return value with the current match state using the make function, which works something like a return, but doesn't clobber the match state:
$str ~~ / foo # Match 'foo'
{ make 'bar' } # But pretend we matched 'bar'
/;
say $(); # says 'bar'
# subpattern
# _________________/\___________________
# | |
# | subpattern subpattern |
# | __/\__ __/\__ |
# | | | | | |
mm/ (I am the (walrus), ( khoo )**2 kachoo) /;
Match object if it is successfully matched.Match object is pushed onto the array inside the outer Match object belonging to the surrounding scope (known as its parent Match object). The surrounding scope may be either the innermost surrounding subpattern (if the subpattern is nested) or else the entire regex itself. # subpat-A
# _________________/\__________________
# | |
# | subpat-B subpat-C |
# | __/\__ __/\__ |
# | | | | | |
mm/ (I am the (walrus), ( khoo )**2 kachoo) /;
then the Match objects representing the matches made by subpat-B and subpat-C would be successively pushed onto the array inside subpat- A's Match object. Then subpat-A's Match object would itself be pushed onto the array inside the Match object for the entire regex (i.e. onto $/'s array).
Match object are referred to using either the standard array access notation (e.g. $/[0], $/[1], $/[2], etc.) or else via the corresponding lexically scoped numeric aliases (i.e. $0, $1, $2, etc.) So:
say "$/[1] was found between $/[0] and $/[2]";
is the same as:
say "$1 was found between $0 and $2";
$/.Match object (i.e. $/) store individual Match objects representing the substrings that were matched and captured by the first, second, third, etc. outermost (i.e. unnested) subpatterns. So these elements can be treated like fully fledged match results. For example:
if m/ (\d\d\d\d)-(\d\d)-(\d\d) (BCE?|AD|CE)?/ {
($yr, $mon, $day) = $/[0..2];
$era = "$3" if $3; # stringify/boolify
@datepos = ( $0.from() .. $2.to() ); # Call Match methods
}
Match object, not to the array of $/. # Perl 5...
#
# $1--------------------- $4--------- $5------------------
# | $2--------------- | | | | $6---- $7------ |
# | | $3-- | | | | | | | | | |
# | | | | | | | | | | | | | |
m/ ( A (guy|gal|g(\S+) ) ) (sees|calls) ( (the|a) (gal|guy) ) /x;
# Perl 6...
#
# $0--------------------- $1--------- $2------------------
# | $0[0]------------ | | | | $2[0]- $2[1]--- |
# | | $0[0][0] | | | | | | | | | |
# | | | | | | | | | | | | | |
m/ ( A (guy|gal|g(\S+) ) ) (sees|calls) ( (the|a) (gal|guy) ) /;
Match object. Instead, it produces a list of Match objects corresponding to the sequence of individual matches made by the repeated subpattern.Match objects, the corresponding array element for the quantified capture will store a (nested) array rather than a single Match object. For example:
if m/ (\w+) \: (\w+ \s+)* / {
say "Key: $0"; # Unquantified --> single Match
say "Values: @($1)"; # Quantified --> array of Match
}
# non-capturing quantifier
# __________/\____________ __/\__
# | || |
# | $0 $1 || |
# | _^_ ___^___ || |
# | | | | | || |
m/ [ (\w+) \: (\w+ \h*)* \n ] ** 2..* /
Non-capturing brackets don't create a separate nested lexical scope, so the two subpatterns inside them are actually still in the regex's top-level scope, hence their top-level designations: $0 and $1.
$0 and $1 will each contain an array. The elements of that array will be the submatches returned by the corresponding subpatterns on each iteration of the non-capturing parentheses. For example:
my $text = "foo:food fool\nbar:bard barb";
# $0-- $1------
# | | | |
$text ~~ m/ [ (\w+) \: (\w+ \h*)* \n ] ** 2..* /;
# Because they're in a quantified non-capturing block...
# $0 contains the equivalent of:
#
# [ Match.new(str=>'foo'), Match.new(str=>'bar') ]
#
# and $1 contains the equivalent of:
#
# [ Match.new(str=>'food '),
# Match.new(str=>'fool' ),
# Match.new(str=>'bard '),
# Match.new(str=>'barb' ),
# ]
Match objects representing the captures of the inner parens for every iteration (as described above). That is:
my $text = "foo:food fool\nbar:bard barb";
# $0-----------------------
# | |
# | $0[0] $0[1]--- |
# | | | | | |
$text ~~ m/ ( (\w+) \: (\w+ \h*)* \n ) ** 2..* /;
# Because it's in a quantified capturing block,
# $0 contains the equivalent of:
#
# [ Match.new( str=>"foo:food fool\n",
# arr=>[ Match.new(str=>'foo'),
# [
# Match.new(str=>'food '),
# Match.new(str=>'fool'),
# ]
# ],
# ),
# Match.new( str=>'bar:bard barb',
# arr=>[ Match.new(str=>'bar'),
# [
# Match.new(str=>'bard '),
# Match.new(str=>'barb'),
# ]
# ],
# ),
# ]
#
# and there is no $1
| or || (but not after each & or &&). Hence:
# $0 $1 $2 $3 $4 $5
$tune_up = rx/ ("don't") (ray) (me) (for) (solar tea), ("d'oh!")
# $0 $1 $2 $3 $4
| (every) (green) (BEM) (devours) (faces)
/;
This means that if the second alternation matches, the @$/ array will contain ('every', 'green', 'BEM', 'devours', 'faces') rather than (undef, undef, undef, undef, undef, undef, 'every', 'green', 'BEM', 'devours', 'faces') (as the same regex would in Perl 5).
<regex> within a pattern is known as a subrule, whether that regex is actually defined as a regex or token or rule or even an ordinary method or multi. # subrule subrule subrule
# __^__ _______^_____ __^__
# | | | | | |
m/ <ident> $<spaces>=(\s*) <digit>+ /
Match object. But, unlike subpatterns, that Match object is not assigned to the array inside its parent Match object. Instead, it is assigned to an entry of the hash inside its parent Match object. For example:
# .... $/ .....................................
# : :
# : .... $/[0] .................. :
# : : : :
# : $/<ident> : $/[0]<ident> : :
# : __^__ : __^__ : :
# : | | : | | : :
mm/ <ident> \: ( known as <ident> previously ) /
Match object can be referred to using any of the standard hash access notations ($/{'foo'}, $/<bar>, $/«baz», etc.), or else via corresponding lexically scoped aliases ($<foo>, $«bar», $<baz>, etc.) So the previous example also implies:
# $<ident> $0<ident>
# __^__ __^__
# | | | |
mm/ <ident> \: ( known as <ident> previously ) /
<ident>) or aliased internally (<ident=name>) or aliased externally ($<ident>=(<alpha>\w*)). The name's the thing.Match objects rather than a single Match object.Match objects to this array. For example:
if mm/ mv <file> <file> / {
$from = $<file>[0];
$to = $<file>[1];
}
(Note, for clarity we are ignoring whitespace subtleties here--the normal sigspace rules would require space only between alphanumeric characters, which is wrong. Assume that our file subrule deals with whitespace on its own.)
Likewise, with a quantified subrule:
if mm/ mv <file> ** 2 / {
$from = $<file>[0];
$to = $<file>[1];
}
And with a mixture of both:
if mm/ mv <file>+ <file> / {
$to = pop @($<file>);
@from = @($<file>);
}
if mm/ mv <file> <dir=file> / {
$from = $<file>; # Only one subrule named <file>, so scalar
$to = $<dir>; # The Capture Formerly Known As <file>
}
Likewise, neither of the following constructions causes <file> to produce an array of Match objects, since none of them has two or more <file> subrules in the same lexical scope:
if mm/ (keep) <file> | (toss) <file> / {
# Each <file> is in a separate alternation, therefore <file>
# is not repeated in any one scope, hence $<file> is
# not an Array object...
$action = $0;
$target = $<file>;
}
if mm/ <file> \: (<file>|none) / {
# Second <file> nested in subpattern which confers a
# different scope...
$actual = $/<file>;
$virtual = $/[0]<file> if $/[0]<file>;
}
Match object). So:
if mm/ <file> \: [<file>|none] / { # Two <file>s in same scope
$actual = $/<file>[0];
$virtual = $/<file>[1] if $/<file>[1];
}
Aliases can be named or numbered. They can be scalar-, array-, or hash-like. And they can be applied to either capturing or non-capturing constructs. The following sections highlight special features of the semantics of some of those combinations.
# _____/capturing parens\_____
# | |
# | |
mm/ $<key>=( (<[A..E]>) (\d**3..6) (X?) ) /;
then the outer capturing parens no longer capture into the array of $/ as unaliased parens would. Instead the aliased parens capture into the hash of $/; specifically into the hash element whose key is the alias name.
$<key> (i.e. $/<key>), but not $0 (i.e. not $/[0]).$/<key> will contain the Match object that would previously have been placed in $/[0].$/<key>[0] will contain the A-E letter,$/<key>[1] will contain the digits,$/<key>[2] will contain the optional X. # __/non-capturing brackets\__
# | |
# | |
mm/ $<key>=[ (<[A..E]>) (\d**3..6) (X?) ] /;
then the corresponding $/<key> Match object contains only the string matched by the non-capturing brackets.
$/<key> entry is empty. That's because square brackets do not create a nested lexical scope, so the subpatterns are unnested and hence correspond to $0, $1, and $2, and not to $/<key>[0], $/<key>[1], and $/<key>[2].$/<key> will contain the complete substring matched by the square brackets (in a Match object, as described above),$0 will contain the A-E letter,$1 will contain the digits,$2 will contain the optional X.Match object to the hash entry whose key is the name of the alias. And it no longer assigns anything to the hash entry whose key is the subrule name. That is:
if m/ ID\: <id=ident> / {
say "Identified as $/<id>"; # $/<ident> is undefined
}
Match object. This is particularly useful for differentiating two or more calls to the same subrule in the same scope. For example:
if mm/ mv <file>+ <dir=file> / {
@from = @($<file>);
$to = $<dir>;
}
m/ $1=(<-[:]>*) \: $0=<ident> /
the behavior is exactly the same as for a named alias (i.e. the various cases described above), except that the resulting Match object is assigned to the corresponding element of the appropriate array rather than to an element of the hash.
# --$1--- -$2- --$6--- -$7-
# | | | | | | | |
m/ $1=(food) (bard) $6=(bazd) (quxd) /;
$tune_up = rx/ ("don't") (ray) (me) (for) (solar tea), ("d'oh!")
| $6 = (every) (green) (BEM) (devours) (faces)
# $7 $8 $9 $10
/;
# Perl 5...
# $1
# _____________/\___________
# | $2 $3 $4 |
# | __/\___ __/\___ /\ |
# | | | | | | | |
m/ ( ( [A-E] ) (\d{3,6}) (X?) ) /x;
# Perl 6...
# $0
# ______________/\______________
# | $0[0] $0[1] $0[2] |
# | ___/\___ ____/\____ /\ |
# | | | | | | | |
m/ ( (<[A..E]>) (\d ** 3..6) (X?) ) /;
# Perl 6 simulating Perl 5...
# $1
# _______________/\________________
# | $2 $3 $4 |
# | ___/\___ ____/\____ /\ |
# | | | | | | | |
m/ $1=[ (<[A..E]>) (\d ** 3..6) (X?) ] /;
The non-capturing brackets don't introduce a scope, so the subpatterns within them are at regex scope, and hence numbered at the top level. Aliasing the square brackets to $1 means that the next subpattern at the same level (i.e. the (<[A..E]>)) is numbered sequentially (i.e. $2), etc.
Match objects (as described in "Quantified subpattern captures" and "Repeated captures of the same subrule"). So the corresponding array element or hash entry for the alias will contain an array, instead of a single Match object. if mm/ mv $0=<file>+ / {
# <file>+ returns a list of Match objects,
# so $0 contains an array of Match objects,
# one for each successful call to <file>
# $/<file> does not exist (it's pre-empted by the alias)
}
if m/ mv \s+ $<from>=(\S+ \s+)* / {
# Quantified subpattern returns a list of Match objects,
# so $/<from> contains an array of Match
# objects, one for each successful match of the subpattern
# $0 does not exist (it's pre-empted by the alias)
}
Match object which contains only the complete substring that was matched by the full set of repetitions of the brackets (as described in "Named scalar aliases applied to non-capturing brackets"). For example:
"coffee fifo fumble" ~~ m/ $<effs>=[f <-[f]> ** 1..2 \s*]+ /;
say $<effs>; # prints "fee fifo fum"
m/ mv \s+ @<from>=[(\S+) \s+]* <dir> /;
@alias= notation instead of a $alias= mandates that the corresponding hash entry or array element always receives an array of Match objects, even if the construct being aliased would normally return a single Match object. This is useful for creating consistent capture semantics across structurally different alternations (by enforcing array captures in all branches):
mm/ Mr?s? @<names>=<ident> W\. @<names>=<ident>
| Mr?s? @<names>=<ident>
/;
# Aliasing to @names means $/<names> is always
# an Array object, so...
say @($/<names>);
@<key> can also be used outside a regex, as a shorthand for @( $/<key> ). That is:
mm/ Mr?s? @<names>=<ident> W\. @<names>=<ident>
| Mr?s? @<names>=<ident>
/;
say @<names>;
mm/ mv $<files>=[ f.. \s* ]* /; # $/<files> assigned a single
# Match object containing the
# complete substring matched by
# the full set of repetitions
# of the non-capturing brackets
mm/ mv @<files>=[ f.. \s* ]* /; # $/<files> assigned an array,
# each element of which is a
# Match object containing
# the substring matched by Nth
# repetition of the non-
# capturing bracket match
Match object returned by one repetition of the subpattern. That is, an array alias on a subpattern flattens and collects all nested subpattern captures within the aliased subpattern. For example:
if mm/ $<pairs>=( (\w+) \: (\N+) )+ / {
# Scalar alias, so $/<pairs> is assigned an array
# of Match objects, each of which has its own array
# of two subcaptures...
for @($<pairs>) -> $pair {
say "Key: $pair[0]";
say "Val: $pair[1]";
}
}
if mm/ @<pairs>=( (\w+) \: (\N+) )+ / {
# Array alias, so $/<pairs> is assigned an array
# of Match objects, each of which is flattened out of
# the two subcaptures within the subpattern
for @($<pairs>) -> $key, $val {
say "Key: $key";
say "Val: $val";
}
}
Match object returned by each repetition of the subrule, all flattened into a single array:
rule pair { (\w+) \: (\N+) \n }
if mm/ $<pairs>=<pair>+ / {
# Scalar alias, so $/<pairs> contains an array of
# Match objects, each of which is the result of the
# <pair> subrule call...
for @($<pairs>) -> $pair {
say "Key: $pair[0]";
say "Val: $pair[1]";
}
}
if mm/ mv @<pairs>=<pair>+ / {
# Array alias, so $/<pairs> contains an array of
# Match objects, all flattened down from the
# nested arrays inside the Match objects returned
# by each match of the <pair> subrule...
for @($<pairs>) -> $key, $val {
say "Key: $key";
say "Val: $val";
}
}
Match objects is assigned into the appropriate element of the regex's match array rather than to a key of its match hash. For example:
if m/ mv \s+ @0=((\w+) \s+)+ $1=((\W+) (\s*))* / {
# | |
# | |
# | \_ Scalar alias, so $1 gets an
# | array, with each element
# | a Match object containing
# | the two nested captures
# |
# \___ Array alias, so $0 gets a flattened array of
# just the (\w+) captures from each repetition
@from = @($0); # Flattened list
$to_str = $1[0][0]; # Nested elems of
$to_gap = $1[0][1]; # unflattened list
}
@0 is simply a shorthand for @($0), so the first assignment above could also have been written:
@from = @0;
m/ mv %<location>=( (<ident>) \: (\N+) )+ /;
Match object to be assigned a (nested) Hash object (rather than an Array object or a single Match object).