Onnowpurbo: New page: ====POSIX Basic Regular Expressions==== Traditional Unix regular expression syntax followed common conventions but often differed from tool to tool. The [[Institute of Electrical and E...

2008-03-12T04:30:05Z

New page: ====POSIX Basic Regular Expressions==== Traditional Unix regular expression syntax followed common conventions but often differed from tool to tool. The [[Institute of Electrical and E...

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====POSIX Basic Regular Expressions====
Traditional [[Unix]] regular expression syntax followed common conventions but often differed from tool to tool. The [[Institute of Electrical and Electronics Engineers|IEEE]] [[POSIX]] Basic Regular Expressions (BRE) standard (released alongside an alternative flavor called Extended Regular Expressions or ERE) was designed mostly for backward compatibility with the traditional syntax but provided a common standard which has since been adopted as the default syntax of many Unix regular expression tools, though there is often some variation or additional features. Many such tools also provide support for ERE syntax with [[command line argument]]s.

In the BRE syntax, most characters are treated as [[literal]]s — they match only themselves (i.e., <code>a</code> matches "''a''"). The exceptions, listed below, are called [[metacharacter]]s or metasequences.

{|
|- valign="top"
|<code>.</code>
|Matches any single character except newlines (exactly which characters are considered newlines is flavor, character encoding, and platform specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, <code>a.c</code> matches "''abc''", etc., but <code>[a.c]</code> matches only "''a''", "''.''", or "''c''".
|- valign="top"
|<code>[ ]</code>
|A bracket expression. Matches a single character that is contained within the brackets. For example, <code>[abc]</code> matches "''a''", "''b''", or "''c''". <code>[a-z]</code> specifies a range which matches any lowercase letter from "''a''" to "''z''". These forms can be mixed: <code>[abcx-z]</code> matches "''a''", "''b''", "''c''", "''x''", "''y''", and "''z''", as does <code>[a-cx-z]</code>.
The <code>-</code> character is treated as a literal character if it is the last or the first character within the brackets, or if it is escaped with a backslash: <code>[abc-]</code>, <code>[-abc]</code>, or <code>[a\-bc]</code>.
|- valign="top"
|<code>[^ ]</code>
|Matches a single character that is not contained within the brackets. For example, <code>[^abc]</code> matches any character other than "''a''", "''b''", or "''c''". <code>[^a-z]</code> matches any single character that is not a lowercase letter from "''a''" to "''z''". As above, literal characters and ranges can be mixed.
|- valign="top"
|<code>^</code>
|Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
|- valign="top"
|<code>$</code>
|Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
|- valign="top"
|<code></code>
|Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, <code>\''n''</code>). A marked subexpression is also called a block or capturing group.
|- valign="top"
|<code>\''n''</code>
|Matches what the ''n''th marked subexpression matched, where ''n'' is a digit from 1 to 9. This construct is theoretically '''irregular''' and was not adopted in the POSIX ERE syntax. Some tools allow referencing more than nine capturing groups.
|- valign="top"
|<code>*</code>
|Matches the preceding element zero or more times. For example, <code>ab*c</code> matches "''ac''", "''abc''", "''abbbc''", etc. <code>[xyz]*</code> matches "", "''x''", "''y''", "''z''", "''zx''", "''zyx''", "''xyzzy''", and so on. <code>$ab$*</code> matches "", "''ab''", "''abab''", "''ababab''", and so on.
|- valign="top"
|<code>\{''m'',''n''\}</code>
|Matches the preceding element at least ''m'' and not more than ''n'' times. For example, <code>a\{3,5\}</code> matches only "''aaa''", "''aaaa''", and "''aaaaa''". This is not found in a few, older instances of regular expressions.
|}

'''Examples:'''
*<code>.at</code> matches any three-character string ending with "at", including "''hat''", "''cat''", and "''bat''".
*<code>[hc]at</code> matches "''hat''" and "''cat''".
*<code>[^b]at</code> matches all strings matched by <code>.at</code> except "''bat''".
*<code>^[hc]at</code> matches "''hat''" and "''cat''", but only at the beginning of the string or line.
*<code>[hc]at$</code> matches "''hat''" and "''cat''", but only at the end of the string or line.

====POSIX Extended Regular Expressions====
The meaning of metacharacters [[escape sequence|escaped]] with a backslash is reversed for some characters in the POSIX Extended Regular Expression (ERE) syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. Additionally, support is removed for <code>\<em>n</em></code> backreferences and the following metacharacters are added:

{|
|-
|<code>?</code>
|Matches the preceding element zero or one time. For example, <code>ba?</code> matches "''b''" or "''ba''".
|-
|<code>+</code>
|Matches the preceding element one or more times. For example, <code>ba+</code> matches "''ba''", "''baa''", "''baaa''", and so on.
|-
|<code><nowiki>|</nowiki></code>
|The choice (aka alternation or set union) operator matches either the expression before or the expression after the operator. For example, <code><nowiki>abc|def</nowiki></code> matches "''abc''" or "''def''".</td>
|}

'''Examples:'''
*<code>[hc]+at<code> matches "''hat''", "''cat''", "''hhat''", "''chat''", "''hcat''", "''ccchat''", and so on, but not "''at''".
*<code>[hc]?at</code> matches "''hat''", "''cat''", and "''at''".
*<code>cat|dog</code> matches "''cat''" or "''dog''".

POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the [[command line]] flag <var>-E</var>.

====POSIX character classes====
Since many ranges of characters depend on the chosen locale setting (i.e., in some settings letters are organized as ''abc...zABC...Z'', while in some others as ''aAbBcC...zZ''), the POSIX standard defines some classes or categories of characters as shown in the following table:

{| class="wikitable"
|-
! POSIX !! ASCII !! Description
|-
| <code>[:alnum:]</code>
| <code>[A-Za-z0-9]</code>
| Alphanumeric characters
|-
| <code>[:alpha:]</code>
| <code>[A-Za-z]</code>
| Alphabetic characters
|-
| <code>[:blank:]</code>
| <code>[ \t]</code>
| Space and tab
|-
| <code>[:cntrl:]</code>
| <code>[\x00-\x1F\x7F]</code>
| Control characters
|-
| <code>[:digit:]</code>
| <code>[0-9]</code>
| Digits
|-
| <code>[:graph:]</code>
| <code>[\x21-\x7E]</code>
| Visible characters
|-
| <code>[:lower:]</code>
| <code>[a-z]</code>
| Lowercase letters
|-
| <code>[:print:]</code>
| <code>[\x20-\x7E]</code>
| Visible characters and spaces
|-
| <code>[:punct:]</code>
| <code><nowiki>[!"#$%&'()*+,-./:;<=>?@[\\\]_`{|}~]</nowiki></code>
| Punctuation characters
|-
| <code>[:space:]</code>
| <code>[ \t\r\n\v\f]</code>
| Whitespace characters
|-
| <code>[:upper:]</code>
| <code>[A-Z]</code>
| Uppercase letters
|-
| <code>[:xdigit:]</code>
| <code>[A-Fa-f0-9]</code>
| Hexadecimal digits
|}

POSIX character classes can only be used within bracket expressions. For example, <code><nowiki>[[:upper:]ab]</nowiki></code> matches the uppercase letters and lowercase "''a''" and "''b''".

In Perl regular expressions, <code>[:print:]</code> matches <code>[:graph:]</code> union <code>[:space:]</code>. An additional non-POSIX class understood by some tools is <code>[:word:]</code>, which is usually defined as <code>[:alnum:]</code> plus underscore. This reflects the fact that in many programming languages these are the characters that may be used in identifiers. The editor [[Vim (text editor)|Vim]] further distinguishes ''word'' and ''word-head'' classes (using the notation <code>\w</code> and <code>\h</code>) since in many programming languages the characters that can begin an identifier are not the same as those that can occur in other positions.

Note that what the POSIX regular expression standards call ''character classes'' are commonly referred to as ''POSIX character classes'' in other regular expression flavors which support them. With most other regular expression flavors, the term ''character class'' is used to describe what POSIX calls ''bracket expressions''.

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Onnowpurbo: New page: ====POSIX Basic Regular Expressions==== Traditional Unix regular expression syntax followed common conventions but often differed from tool to tool. The [[Institute of Electrical and E...