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### POSIX Basic Regular Expression Syntax

#### Synopsis

The POSIX-Basic regular expression syntax is used by the Unix utility `sed`, and variations are used by `grep` and `emacs`. You can construct POSIX basic regular expressions in Boost.Regex by passing the flag `basic` to the regex constructor (see `syntax_option_type`), for example:

    // e1 is a case sensitive POSIX-Basic expression:
    boost::regex e1(my_expression, boost::regex::basic);
    // e2 a case insensitive POSIX-Basic expression:
    boost::regex e2(my_expression, boost::regex::basic|boost::regex::icase);

#### POSIX Basic Syntax

In POSIX-Basic regular expressions, all characters are 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 when used as the first character of an expression, or the first character of a sub-expression.

A '$' character shall match the end of a line when used as the last character of an expression, or the last character of a sub-expression.

##### 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.

##### Repeats:

Any atom (a single character, a marked sub-expression, or a character class) can be repeated with the * operator.

For example `a*` will match any number of letter a's repeated zero or more times (an atom repeated zero times matches an empty string), so the expression `a*b` will match any of the following:

    b
    ab
    aaaaaaaab

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.

##### 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

##### 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-Basic 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 when constructing the regular expression - 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 rangle "ae"-c, assuming that "ae" is treated as a single collating element in the current locale.

Collating elements may be used in place of escapes (which are not normally allowed inside character sets), for example `[[.^.]abc]` would match either one of the characters 'abc^'.

As an extension, a collating element may also be specified via its symbolic name, for example:

    [[.NUL.]]

matches a 'NUL' character. See collating element names.

###### Equivalence classes:

An expression of theform `[[=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 collating 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.

###### Combinations:

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

##### Escapes

With the exception of the escape sequences \{, \}, \(, and \), which are documented above, an escape followed by any character matches that character. This can be used to make the special characters

    .[\*^$

"ordinary". Note that the escape character loses its special meaning inside a character set, so `[\^]` will match either a literal '\' or a '^'.

#### What Gets Matched

When there is more that one way to match a regular expression, the "best" possible match is obtained using the leftmost-longest rule.

#### Variations

##### Grep

When an expression is compiled with the flag `grep` set, then the expression is treated as a newline separated list of POSIX-Basic expressions, a match is found if any of the expressions in the list match, for example:

    boost::regex e("abc\ndef", boost::regex::grep);

will match either of the POSIX-Basic expressions "abc" or "def".

As its name suggests, this behavior is consistent with the Unix utility grep.

##### emacs

In addition to the POSIX-Basic features the following characters are also special:

Character | Description
--- | ---
+ | repeats the preceding atom one or more times.
? | repeats the preceding atom zero or one times.
*? | A non-greedy version of *.
+? | A non-greedy version of +.
?? | A non-greedy version of ?.

And the following escape sequences are also recognised:

Escape | Description
--- | ---
\| | specifies an alternative.
\(?: ... ) | is a non-marking grouping construct - allows you to lexically group something without spitting out an extra sub-expression.
\w | matches any word character.
\W | matches any non-word character.
\sx | matches any character in the syntax group x, the following emacs groupings are supported: 's', ' ', '_', 'w', '.', ')', '(', '"', '\'', '>' and '<'. Refer to the emacs docs for details.
\Sx | matches any character not in the syntax grouping x.
\c and \C | These are not supported.
\` | matches zero characters only at the start of a buffer (or string being matched).
\' | matches zero characters only at the end of a buffer (or string being matched).
\b | matches zero characters at a word boundary.
\B | matches zero characters, not at a word boundary.
\< | matches zero characters only at the start of a word.
\> | matches zero characters only at the end of a word.

Finally, you should note that emacs style regular expressions are matched according to the Perl "depth first search" rules. Emacs expressions are matched this way because they contain Perl-like extensions, that do not interact well with the POSIX-style leftmost-longest rule.

#### Options

There are a variety of flags that may be combined with the `basic` and `grep` options when constructing the regular expression, in particular note that the `newline_alt`, `no_char_classes`, `no-intervals`, `bk_plus_qm` and `bk_plus_vbar` options all alter the syntax, while the `collate` and `icase` options modify how the case and locale sensitivity are to be applied.

#### References

IEEE Std 1003.1-2001, Portable Operating System Interface (POSIX ), Base Definitions and Headers, Section 9, Regular Expressions (FWD.1).

IEEE Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and Utilities, Section 4, Utilities, grep (FWD.1).

Emacs Version 21.3.