[/ 
  Copyright 2006-2007 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]


[section:perl_syntax Perl Regular Expression Syntax]

[h3 Synopsis]

The Perl regular expression syntax is based on that used by the 
programming language Perl .  Perl regular expressions are the 
default behavior in Boost.Regex or you can pass the flag =perl= to the 
[basic_regex] constructor, for example:

   // e1 is a case sensitive Perl regular expression: 
   // since Perl is the default option there's no need to explicitly specify the syntax used here:
   boost::regex e1(my_expression);
   // e2 a case insensitive Perl regular expression:
   boost::regex e2(my_expression, boost::regex::perl|boost::regex::icase);

[h3 Perl Regular Expression Syntax]

In Perl regular expressions, all characters match themselves except for the 
following special characters:

[pre .\[{}()\\\*+?|^$]

[h4 Wildcard]

The single character '.' when used outside of a character set will match 
any single character except:

* The NULL character when the [link boost_regex.ref.match_flag_type flag 
   =match_not_dot_null=] is passed to the matching algorithms.
* The newline character when the [link boost_regex.ref.match_flag_type 
   flag =match_not_dot_newline=] is passed to 
   the matching algorithms.
   
[h4 Anchors]

A '^' character shall match the start of a line.

A '$' character shall match the end of a line.

[h4 Marked sub-expressions]

A section beginning =(= and ending =)= acts as a marked sub-expression.  
Whatever matched the sub-expression is split out in a separate field by 
the matching algorithms.  Marked sub-expressions can also repeated, or 
referred to by a back-reference.

[h4 Non-marking grouping]

A marked sub-expression is useful to lexically group part of a regular 
expression, but has the side-effect of spitting out an extra field in 
the result.  As an alternative you can lexically group part of a 
regular expression, without generating a marked sub-expression by using 
=(?:= and =)= , for example =(?:ab)+= will repeat =ab= without splitting 
out any separate sub-expressions.

[h4 Repeats]

Any atom (a single character, a marked sub-expression, or a character class) 
can be repeated with the =*=, =+=, =?=, and ={}= operators.

The =*= operator will match the preceding atom zero or more times, 
for example the expression =a*b= will match any of the following:

   b
   ab
   aaaaaaaab

The =+= operator will match the preceding atom one or more times, for 
example the expression =a+b= will match any of the following:

   ab
   aaaaaaaab

But will not match:

   b

The =?= operator will match the preceding atom zero or one times, for 
example the expression ca?b will match any of the following:

   cb
   cab

But will not match:

   caab

An atom can also be repeated with a bounded repeat:

=a{n}=  Matches 'a' repeated exactly n times.

=a{n,}=  Matches 'a' repeated n or more times.

=a{n, m}=  Matches 'a' repeated between n and m times inclusive.

For example:

[pre ^a{2,3}$]

Will match either of:

   aa
   aaa

But neither of:

   a
   aaaa

It is an error to use a repeat operator, if the preceding construct can not 
be repeated, for example:

   a(*)

Will raise an error, as there is nothing for the =*= operator to be applied to.

[h4 Non greedy repeats]

The normal repeat operators are "greedy", that is to say they will consume as 
much input as possible.  There are non-greedy versions available that will 
consume as little input as possible while still producing a match.

=*?= Matches the previous atom zero or more times, while consuming as little 
   input as possible.

=+?= Matches the previous atom one or more times, while consuming as 
   little input as possible.

=??= Matches the previous atom zero or one times, while consuming 
   as little input as possible.

={n,}?= Matches the previous atom n or more times, while consuming as 
   little input as possible.

={n,m}?= Matches the previous atom between n and m times, while 
   consuming as little input as possible.
   
[h4 Possessive repeats]

By default when a repeated pattern does not match then the engine will backtrack until
a match is found.  However, this behaviour can sometime be undesireable so there are
also "possessive" repeats: these match as much as possible and do not then allow
backtracking if the rest of the expression fails to match.

=*+= Matches the previous atom zero or more times, while giving nothing back.

=++= Matches the previous atom one or more times, while giving nothing back.

=?+= Matches the previous atom zero or one times, while giving nothing back.

={n,}+= Matches the previous atom n or more times, while giving nothing back.

={n,m}+= Matches the previous atom between n and m times, while giving nothing back.
   
[h4 Back references]

An escape character followed by a digit /n/, where /n/ is in the range 1-9, 
matches the same string that was matched by sub-expression /n/.  For example 
the expression:

[pre ^(a\*).\*\\1$]

Will match the string:

   aaabbaaa

But not the string:

   aaabba
   
You can also use the \g escape for the same function, for example:

[table
[[Escape][Meaning]]
[[=\g1=][Match whatever matched sub-expression 1]]
[[=\g{1}=][Match whatever matched sub-expression 1: this form allows for safer 
	parsing of the expression in cases like =\g{1}2= or for indexes higher than 9 as in =\g{1234}=]]
[[=\g-1=][Match whatever matched the last opened sub-expression]]
[[=\g{-2}=][Match whatever matched the last but one opened sub-expression]]
[[=\g{one}=][Match whatever matched the sub-expression named "one"]]
]

Finally the \k escape can be used to refer to named subexpressions, for example [^\k<two>] will match
whatever matched the subexpression named "two".

[h4 Alternation]

The =|= operator will match either of its arguments, so for example: 
=abc|def= will match either "abc" or "def". 

Parenthesis can be used to group alternations, for example: =ab(d|ef)= 
will match either of "abd" or "abef".

Empty alternatives are not allowed (these are almost always a mistake), but 
if you really want an empty alternative use =(?:)= as a placeholder, for example:

=|abc= is not a valid expression, but

=(?:)|abc= is and is equivalent, also the expression:

=(?:abc)??= has exactly the same effect.

[h4 Character sets]

A character set is a bracket-expression starting with =[= and ending with =]=, 
it defines a set of characters, and matches any single character that is a 
member of that set.

A bracket expression may contain any combination of the following:

[h5 Single characters]

For example =[abc]=, will match any of the characters 'a', 'b', or 'c'.

[h5 Character ranges]

For example =[a-c]= will match any single character in the range 'a' to 'c'.  
By default, for Perl regular expressions, a character x is within the 
range y to z, if the code point of the character lies within the codepoints of
the endpoints of the range.  Alternatively, if you set the 
[link boost_regex.ref.syntax_option_type.syntax_option_type_perl =collate= flag] 
when constructing the regular expression, then ranges are locale sensitive.

[h5 Negation]

If the bracket-expression begins with the ^ character, then it matches the 
complement of the characters it contains, for example =[^a-c]= matches 
any character that is not in the range =a-c=.

[h5 Character classes]

An expression of the form [^\[\[:name:\]\]] matches the named character class 
"name", for example [^\[\[:lower:\]\]] matches any lower case character.  
See [link boost_regex.syntax.character_classes character class names].

[h5 Collating Elements]

An expression of the form [^\[\[.col.\]\]] matches the collating element /col/.  
A collating element is any single character, or any sequence of characters 
that collates as a single unit.  Collating elements may also be used 
as the end point of a range, for example: [^\[\[.ae.\]-c\]] matches the 
character sequence "ae", plus any single character in the range "ae"-c, 
assuming that "ae" is treated as a single collating element in the current locale.

As an extension, a collating element may also be specified via it's 
[link boost_regex.syntax.collating_names symbolic name], for example:

   [[.NUL.]]

matches a =\0= character.

[h5 Equivalence classes]

An expression of the form [^\[\[\=col\=\]\]], matches any character or collating element 
whose primary sort key is the same as that for collating element /col/, as with 
collating elements the name /col/ may be a 
[link boost_regex.syntax.collating_names symbolic name].  A primary sort key is 
one that ignores case, accentation, or locale-specific tailorings; so for 
example `[[=a=]]` matches any of the characters: 
a, '''&#xC0;''', '''&#xC1;''', '''&#xC2;''', 
'''&#xC3;''', '''&#xC4;''', '''&#xC5;''', A, '''&#xE0;''', '''&#xE1;''', 
'''&#xE2;''', '''&#xE3;''', '''&#xE4;''' and '''&#xE5;'''.  
Unfortunately implementation of this is reliant on the platform's collation 
and localisation support; this feature can not be relied upon to work portably 
across all platforms, or even all locales on one platform.

[h5 Escaped Characters]

All the escape sequences that match a single character, or a single character 
class are permitted within a character class definition.  For example
`[\[\]]` would match either of `[` or `]` while `[\W\d]` would match any character
that is either a "digit", /or/ is /not/ a "word" character.

[h5 Combinations]

All of the above can be combined in one character set declaration, for example: 
[^\[\[:digit:\]a-c\[.NUL.\]\]].

[h4 Escapes]

Any special character preceded by an escape shall match itself.

The following escape sequences are all synonyms for single characters:

[table
[[Escape][Character]]
[[=\a=][=\a=]]
[[=\e=][=0x1B=]]
[[=\f=][=\f=]]
[[=\n=][=\n=]]
[[=\r=][=\r=]]
[[=\t=][=\t=]]
[[=\v=][=\v=]]
[[=\b=][=\b= (but only inside a character class declaration).]]
[[=\cX=][An ASCII escape sequence - the character whose code point is X % 32]]
[[=\xdd=][A hexadecimal escape sequence - matches the single character whose 
      code point is 0xdd.]]
[[=\x{dddd}=][A hexadecimal escape sequence - matches the single character whose 
      code point is 0xdddd.]]
[[=\0ddd=][An octal escape sequence - matches the single character whose 
   code point is 0ddd.]]
[[=\N{name}=][Matches the single character which has the 
      [link boost_regex.syntax.collating_names symbolic name] /name/.  
      For example =\N{newline}= matches the single character \\n.]]
]      
 
[h5 "Single character" character classes:]

Any escaped character /x/, if /x/ is the name of a character class shall 
match any character that is a member of that class, and any 
escaped character /X/, if /x/ is the name of a character class, shall 
match any character not in that class.

The following are supported by default:

[table
[[Escape sequence][Equivalent to]]
[[`\d`][`[[:digit:]]`]]
[[`\l`][`[[:lower:]]`]]
[[`\s`][`[[:space:]]`]]
[[`\u`][`[[:upper:]]`]]
[[`\w`][`[[:word:]]`]]
[[`\h`][Horizontal whitespace]]
[[`\v`][Vertical whitespace]]
[[`\D`][`[^[:digit:]]`]]
[[`\L`][`[^[:lower:]]`]]
[[`\S`][`[^[:space:]]`]]
[[`\U`][`[^[:upper:]]`]]
[[`\W`][`[^[:word:]]`]]
[[`\H`][Not Horizontal whitespace]]
[[`\V`][Not Vertical whitespace]]
]

[h5 Character Properties]

The character property names in the following table are all equivalent 
to the [link boost_regex.syntax.character_classes names used in character classes].

[table
[[Form][Description][Equivalent character set form]]
[[`\pX`][Matches any character that has the property X.][`[[:X:]]`]]
[[`\p{Name}`][Matches any character that has the property Name.][`[[:Name:]]`]]
[[`\PX`][Matches any character that does not have the property X.][`[^[:X:]]`]]
[[`\P{Name}`][Matches any character that does not have the property Name.][`[^[:Name:]]`]]
]

For example =\pd= matches any "digit" character, as does =\p{digit}=.

[h5 Word Boundaries]

The following escape sequences match the boundaries of words:

=\<= 	Matches the start of a word.

=\>= 	Matches the end of a word.

=\b= 	Matches a word boundary (the start or end of a word).

=\B= 	Matches only when not at a word boundary.

[h5 Buffer boundaries]

The following match only at buffer boundaries: a "buffer" in this 
context is the whole of the input text that is being matched against 
(note that ^ and $ may match embedded newlines within the text).

\\\` 	Matches at the start of a buffer only.

\\' 	Matches at the end of a buffer only.

\\A 	Matches at the start of a buffer only (the same as =\\\`=).

\\z 	Matches at the end of a buffer only (the same as =\\'=).

\\Z 	Matches a zero-width assertion consisting of an optional sequence of newlines at the end of a buffer: 
equivalent to the regular expression [^(?=\\v*\\z)].  Note that this is subtly different from Perl which
behaves as if matching [^(?=\\n?\\z)].

[h5 Continuation Escape]

The sequence =\G= matches only at the end of the last match found, or at 
the start of the text being matched if no previous match was found.  
This escape useful if you're iterating over the matches contained within a 
text, and you want each subsequence match to start where the last one ended.

[h5 Quoting escape]

The escape sequence =\Q= begins a "quoted sequence": all the subsequent characters 
are treated as literals, until either the end of the regular expression or \\E 
is found.  For example the expression: =\Q\*+\Ea+= would match either of:

    \*+a
    \*+aaa

[h5 Unicode escapes]

=\C= 	Matches a single code point: in Boost regex this has exactly the 
   same effect as a "." operator.
=\X= 	Matches a combining character sequence: that is any non-combining 
      character followed by a sequence of zero or more combining characters.
      
[h5 Matching Line Endings]

The escape sequence =\R= matches any line ending character sequence, specifically it is identical to
the expression [^(?>\x0D\x0A?|\[\x0A-\x0C\x85\x{2028}\x{2029}\])].
      
[h5 Keeping back some text]

=\K= Resets the start location of $0 to the current text position: in other words everything to the
left of \K is "kept back" and does not form part of the regular expression match. $` is updated
accordingly.

For example =foo\Kbar= matched against the text "foobar" would return the match "bar" for $0 and "foo"
for $`.  This can be used to simulate variable width lookbehind assertions.
    
[h5 Any other escape]

Any other escape sequence matches the character that is escaped, for example 
\\@ matches a literal '@'.

[h4 Perl Extended Patterns]

Perl-specific extensions to the regular expression syntax all start with =(?=.

[h5 Named Subexpressions]

You can create a named subexpression using:

	(?<NAME>expression)
	
Which can be then be refered to by the name /NAME/.  Alternatively you can delimit the name
using 'NAME' as in:

	(?'NAME'expression)
	
These named subexpressions can be refered to in a backreference using either [^\g{NAME}] or [^\k<NAME>]
and can also be refered to by name in a [perl_format] format string for search and replace operations, or in the
[match_results] member functions.
	
[h5 Comments]

=(?# ... )= is treated as a comment, it's contents are ignored.

[h5 Modifiers]

=(?imsx-imsx ... )= alters which of the perl modifiers are in effect within 
the pattern, changes take effect from the point that the block is first seen 
and extend to any enclosing =)=.  Letters before a '-' turn that perl 
modifier on, letters afterward, turn it off.

=(?imsx-imsx:pattern)= applies the specified modifiers to pattern only.

[h5 Non-marking groups]

=(?:pattern)= lexically groups pattern, without generating an additional 
sub-expression.

[h5 Branch reset]

=(?|pattern)=  resets the subexpression count at the start of each "|" alternative within /pattern/.

The sub-expression count following this construct is that of whichever branch had the largest number of
sub-expressions.  This construct is useful when you want to capture one of a number of alternative matches
in a single sub-expression index. 

In the following example the index of each sub-expression is shown below the expression:

[pre
# before  ---------------branch-reset----------- after        
/ ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1            2         2  3        2     3     4
]

[h5 Lookahead]

[^(?=pattern)] consumes zero characters, only if pattern matches.

=(?!pattern)= consumes zero characters, only if pattern does not match.

Lookahead is typically used to create the logical AND of two regular 
expressions, for example if a password must contain a lower case letter, 
an upper case letter, a punctuation symbol, and be at least 6 characters long, 
then the expression:

    (?=.*[[:lower:]])(?=.*[[:upper:]])(?=.*[[:punct:]]).{6,}

could be used to validate the password.

[h5 Lookbehind]

[^(?<=pattern)] consumes zero characters, only if pattern could be matched 
against the characters preceding the current position (pattern must be 
of fixed length).

=(?<!pattern)= consumes zero characters, only if pattern could not be 
matched against the characters preceding the current position (pattern must 
be of fixed length).

[h5 Independent sub-expressions]

=(?>pattern)= /pattern/ is matched independently of the surrounding patterns, 
the expression will never backtrack into /pattern/.  Independent sub-expressions 
are typically used to improve performance; only the best possible match 
for pattern will be considered, if this doesn't allow the expression as a 
whole to match then no match is found at all.

[h5 Recursive Expressions]

[^(?['N]) (?-['N]) (?+['N]) (?R) (?0)]

=(?R)= and =(?0)= recurse to the start of the entire pattern.

[^(?['N])] executes sub-expression /N/ recursively, for example =(?2)= will recurse to sub-expression 2.

[^(?-['N])] and [^(?+['N])] are relative recursions, so for example =(?-1)= recurses to the last sub-expression to be declared,
and =(?+1)= recurses to the next sub-expression to be declared.

[h5 Conditional Expressions]

=(?(condition)yes-pattern|no-pattern)= attempts to match /yes-pattern/ if 
the /condition/ is true, otherwise attempts to match /no-pattern/.

=(?(condition)yes-pattern)= attempts to match /yes-pattern/ if the /condition/ 
is true, otherwise fails.

/condition/ may be either: a forward lookahead assert, the index of 
a marked sub-expression (the condition becomes true if the sub-expression 
has been matched), or an index of a recursion (the condition become true if we are executing
directly inside the specified recursion).

Here is a summary of the possible predicates:

* [^(?(?\=assert)yes-pattern|no-pattern)]  Executes /yes-pattern/ if the forward look-ahead assert matches, otherwise
executes /no-pattern/.
* =(?(?!assert)yes-pattern|no-pattern)=  Executes /yes-pattern/ if the forward look-ahead assert does not match, otherwise
executes /no-pattern/.
* =(?(R)yes-pattern|no-pattern)=  Executes /yes-pattern/ if we are executing inside a recursion, otherwise
executes /no-pattern/.
* [^(?(R['N])yes-pattern|no-pattern)]  Executes /yes-pattern/ if we are executing inside a recursion to sub-expression /N/, otherwise
executes /no-pattern/.
* [^(?(DEFINE)never-exectuted-pattern)]  Defines a block of code that is never executed and matches no characters:
this is usually used to define one or more named sub-expressions which are refered to from elsewhere in the pattern.

[h4 Operator precedence]

The order of precedence for of operators is as follows:

# Collation-related bracket symbols 	`[==] [::] [..]`
# Escaped characters 	=\=
# Character set (bracket expression) 	`[]`
# Grouping 	=()=
# Single-character-ERE duplication 	=* + ? {m,n}=
# Concatenation 	
# Anchoring 	^$
# Alternation 	|

[h3 What gets matched]

If you view the regular expression as a directed (possibly cyclic) 
graph, then the best match found is the first match found by a 
depth-first-search performed on that graph, while matching the input text.

Alternatively:

The best match found is the 
[link boost_regex.syntax.leftmost_longest_rule leftmost match], 
with individual elements matched as follows;

[table
[[Construct][What gets matched]]
[[=AtomA AtomB=][Locates the best match for /AtomA/ that has a following match for /AtomB/.]]
[[=Expression1 | Expression2=][If /Expresion1/ can be matched then returns that match, 
   otherwise attempts to match /Expression2/.]]
[[=S{N}=][Matches /S/ repeated exactly N times.]]
[[=S{N,M}=][Matches S repeated between N and M times, and as many times as possible.]]
[[=S{N,M}?=][Matches S repeated between N and M times, and as few times as possible.]]
[[=S?, S*, S+=][The same as =S{0,1}=, =S{0,UINT_MAX}=, =S{1,UINT_MAX}= respectively.]]
[[=S??, S*?, S+?=][The same as =S{0,1}?=, =S{0,UINT_MAX}?=, =S{1,UINT_MAX}?= respectively.]]
[[=(?>S)=][Matches the best match for /S/, and only that.]]
[[[^(?=S), (?<=S)]][Matches only the best match for /S/ (this is only 
      visible if there are capturing parenthesis within /S/).]]
[[=(?!S), (?<!S)=][Considers only whether a match for S exists or not.]]
[[=(?(condition)yes-pattern | no-pattern)=][If condition is true, then 
   only yes-pattern is considered, otherwise only no-pattern is considered.]]
]

[h3 Variations]

The [link boost_regex.ref.syntax_option_type.syntax_option_type_perl options =normal=, 
=ECMAScript=, =JavaScript= and =JScript=] are all synonyms for 
=perl=.

[h3 Options]

There are a [link boost_regex.ref.syntax_option_type.syntax_option_type_perl 
variety of flags] that may be combined with the =perl= option when 
constructing the regular expression, in particular note that the 
=newline_alt= option alters the syntax, while the =collate=, =nosubs= and 
=icase= options modify how the case and locale sensitivity are to be applied.

[h3 Pattern Modifiers]

The perl =smix= modifiers can either be applied using a =(?smix-smix)= 
prefix to the regular expression, or with one of the 
[link boost_regex.ref.syntax_option_type.syntax_option_type_perl regex-compile time 
flags =no_mod_m=, =mod_x=, =mod_s=, and =no_mod_s=].

[h3 References]

[@http://perldoc.perl.org/perlre.html Perl 5.8].


[endsect]