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# Boost.Regex

## Perl Regular Expression Syntax

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### Contents

Synopsis
Perl Syntax
What Gets Matched
Variations
Options
Modifiers
References

### 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 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);

### Perl Regular Expression Syntax

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

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

#### Wildcard:

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

The NULL character when the flag match_no_dot_null is passed to the matching algorithms.

The newline character when the flag match_not_dot_newline is passed to the matching algorithms.

#### Anchors:

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

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

#### 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.

#### 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.

#### 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:

    ^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.

#### 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.

#### 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:

    ^(a*).*\1$

Will match the string:

    aaabbaaa

But not the string:

    aaabba

#### 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.

#### 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:

> ##### Single characters:

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

> ##### Character ranges:

> For example [a-c] will match any single character in the range 'a' to 'c'. By default, for POSIX-Perl regular expressions, a character x is within the range y to z, if it collates within that range; this results in locale specific behavior. This behavior can be turned off by unsetting the collate option flag - in which case whether a character appears within a range is determined by comparing the code points of the characters only

> ##### 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.

> ##### Character classes:

> An expression of the form [[:name:]] matches the named character class "name", for example [[:lower:]] matches any lower case character. See character class names.

> ##### 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 symbolic name, for example:

> [[.NUL.]]

> matches a NUL character.

> ##### Equivalence classes:

> An expression oftheform[[=col=]], matches any character or collating element whose primary sort key is the same as that for collating element col, as with colating elements the name col may be a 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, à, á, â, ã, ä, å, A, À, Á, Â, Ã, Ä and Å. 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.

> ##### Escapes:

> All the escape sequences that match a single character, or a single character class are permitted within a character class definition, except the negated character classes (\D \W etc).

##### Combinations:

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

#### Escapes

Any special character preceded by an escape shall match itself.

The following escape sequences are also supported:

> ##### Escapes matching a specific character

> The following escape sequences are all synonyms for single characters:

> 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 symbolic name name. For example \N{newline} matches the single character \n.

> ##### "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:

> Escape sequence | Equivalent to
> \d | [[:digit:]]
> \l | [[:lower:]]
> \s | [[:space:]]
> \u | [[:upper:]]
> \w | [[:word:]]
> \D | [^[:digit:]]
> \L | [^[:lower:]]
> \S | [^[:space:]]
> \U | [^[:upper:]]
> \W | [^[:word:]]

> ##### Character Properties

> The character property names in the following table are all equivalent to the names used in character classes.

> 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:]]

> ##### 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.

> ##### 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 an optional sequence of newlines at the end of a buffer: equivalent to the regular expression \n*\z

> ##### 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.

> ##### 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

> ##### 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.

> ##### Any other escape

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

#### Perl Extended Patterns

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

> ##### Comments

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

> ##### 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.

> ##### Non-marking grouping

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

> ##### 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.

> ##### 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).

> ##### 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.

> ##### 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, or the index of a marked sub-expression (the condition becomes true if the sub-expression has been matched).

#### Operator precedence

The order of precedence for of operators is as shown in the following table:

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

### 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 leftmost match, with individual elements matched as follows;

Construct | What gets matches
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.

### Variations

The options normal, ECMAScript, JavaScript and JScript are all synonyms for Perl.

### Options

There are a 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.

### Modifiers

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

### References

Perl 5.8.

Revised 21 Aug 2004

© Copyright John Maddock 2004

Use, modification and distribution are subject to 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).