Editing Regular expression (section)

====Character classes====
The character class is the most basic regex concept after a literal match. It makes one small sequence of characters match a larger set of characters. For example, <syntaxhighlight lang="ragel" inline>[A-Z]</syntaxhighlight> could stand for any uppercase letter in the English alphabet, and <syntaxhighlight lang="ragel" inline>\d</syntaxhighlight> could mean any digit. Character classes apply to both POSIX levels.

When specifying a range of characters, such as <syntaxhighlight lang="ragel" inline>[a-Z]</syntaxhighlight> (i.e. lowercase ''<syntaxhighlight lang="ragel" inline>a</syntaxhighlight>'' to uppercase ''<syntaxhighlight lang="ragel" inline>Z</syntaxhighlight>''), the computer's locale settings determine the contents by the numeric ordering of the character encoding. They could store digits in that sequence, or the ordering could be ''abc...zABC...Z'', or ''aAbBcC...zZ''. So the POSIX standard defines a character class, which will be known by the regex processor installed. Those definitions are in the following table:

{| class="wikitable sortable"
|-
! Description
! POSIX !! Perl/Tcl !! Vim !! Java !! ASCII
|-
| ASCII characters
|
|
|
| <syntaxhighlight lang="ragel" inline>\p{ASCII}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[\x00-\x7F]</syntaxhighlight>
|-
| Alphanumeric characters
| <syntaxhighlight lang="ragel" inline>[:alnum:]</syntaxhighlight>
|
|
| <syntaxhighlight lang="ragel" inline>\p{Alnum}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[A-Za-z0-9]</syntaxhighlight>
|-
| Alphanumeric characters plus "_"
|
| <syntaxhighlight lang="ragel" inline>\w</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\w</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\w</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[A-Za-z0-9_]</syntaxhighlight>
|-
| Non-word characters
|
| <syntaxhighlight lang="ragel" inline>\W</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\W</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\W</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[^A-Za-z0-9_]</syntaxhighlight>
|-
| Alphabetic characters
| <syntaxhighlight lang="ragel" inline>[:alpha:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\a</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Alpha}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[A-Za-z]</syntaxhighlight>
|-
| Space and tab
| <syntaxhighlight lang="ragel" inline>[:blank:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\s</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Blank}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[ \t]</syntaxhighlight>
|-
| Word boundaries
|
| <syntaxhighlight lang="ragel" inline>\b</syntaxhighlight>
| <code>\&lt; \&gt;</code>
| <syntaxhighlight lang="ragel" inline>\b</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>(?<=\W)(?=\w)|(?<=\w)(?=\W)</syntaxhighlight>
|-
| Non-word boundaries
|
|
|
| <syntaxhighlight lang="ragel" inline>\B</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>(?<=\W)(?=\W)|(?<=\w)(?=\w)</syntaxhighlight>
|-
| [[Control character]]s
| <syntaxhighlight lang="ragel" inline>[:cntrl:]</syntaxhighlight>
|
|
| <syntaxhighlight lang="ragel" inline>\p{Cntrl}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[\x00-\x1F\x7F]</syntaxhighlight>
|-
| Digits
| <syntaxhighlight lang="ragel" inline>[:digit:]</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\d</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\d</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Digit}</syntaxhighlight> or <syntaxhighlight lang="ragel" inline>\d</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[0-9]</syntaxhighlight>
|-
| Non-digits
|
| <syntaxhighlight lang="ragel" inline>\D</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\D</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\D</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[^0-9]</syntaxhighlight>
|-
| Visible characters
| <syntaxhighlight lang="ragel" inline>[:graph:]</syntaxhighlight>
|
|
| <syntaxhighlight lang="ragel" inline>\p{Graph}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[\x21-\x7E]</syntaxhighlight>
|-
| Lowercase letters
| <syntaxhighlight lang="ragel" inline>[:lower:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\l</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Lower}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[a-z]</syntaxhighlight>
|-
| Visible characters and the space character
| <syntaxhighlight lang="ragel" inline>[:print:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\p</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Print}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[\x20-\x7E]</syntaxhighlight>
|-
| Punctuation characters
| <syntaxhighlight lang="ragel" inline>[:punct:]</syntaxhighlight>
|
|
| <syntaxhighlight lang="ragel" inline>\p{Punct}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[][!"#$%&'()*+,./:;<=>?@\^_`{|}~-]</syntaxhighlight>
|-
| [[Whitespace character]]s
| <syntaxhighlight lang="ragel" inline>[:space:]</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\s</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\_s</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Space}</syntaxhighlight> or <syntaxhighlight lang="ragel" inline>\s</syntaxhighlight>
| <code>[ [[\t]][[\r]][[\n]][[\v]][[\f]]]</code>
|-
| Non-whitespace characters
|
| <syntaxhighlight lang="ragel" inline>\S</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\S</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\S</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[^ \t\r\n\v\f]</syntaxhighlight>
|-
| Uppercase letters
| <syntaxhighlight lang="ragel" inline>[:upper:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\u</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{Upper}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[A-Z]</syntaxhighlight>
|-
| Hexadecimal digits
| <syntaxhighlight lang="ragel" inline>[:xdigit:]</syntaxhighlight>
|
| <syntaxhighlight lang="ragel" inline>\x</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>\p{XDigit}</syntaxhighlight>
| <syntaxhighlight lang="ragel" inline>[A-Fa-f0-9]</syntaxhighlight>
|}

POSIX character classes can only be used within bracket expressions. For example, <syntaxhighlight lang="ragel" inline>[[:upper:]ab]</syntaxhighlight> matches the uppercase letters and lowercase "a" and "b".

An additional non-POSIX class understood by some tools is <syntaxhighlight lang="ragel" inline>[:word:]</syntaxhighlight>, which is usually defined as <syntaxhighlight lang="ragel" inline>[:alnum:]</syntaxhighlight> 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 <syntaxhighlight lang="ragel" inline>\w</syntaxhighlight> and <syntaxhighlight lang="ragel" inline>\h</syntaxhighlight>) since in many programming languages the characters that can begin an identifier are not the same as those that can occur in other positions: numbers are generally excluded, so an identifier would look like <syntaxhighlight lang="ragel" inline>\h\w*</syntaxhighlight> or <syntaxhighlight lang="ragel" inline>[[:alpha:]_][[:alnum:]_]*</syntaxhighlight> in POSIX notation.

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