Regular Expression Syntax

Regular expressions or regex are sequences of characters that define a pattern.

There are three types of regular expressions: regex (OS_Regex), sregex (OS_Match) and PCRE2.

Regex (OS_Regex) syntax

This is a fast and simple library for regular expressions in C.

This library is designed to be simple while still supporting the most common regular expressions.

Supported expressions

Expressions	Valid characters
\w	A-Z, a-z, 0-9, ‘-’, ‘@’, ‘_’ characters
\d	0-9 character
\s	Spaces ” “
\t	Tabs
\p	()*+,-.:;<=>?[]!”’#$%&|{}
\W	Anything not \w
\D	Anything not \d
\S	Anything not \s
\.	Anything

Modifiers

Expressions	Actions
+	To match one or more times
*	To match zero or more times

Special characters

Expressions	Actions
^	To specify the beginning of the text
$	To specify the end of the text
|	To create a logical or between multiple patterns

Characters escaping

To utilize the following characters they must be escaped with: \

$	(	)	\	|	<
\$	\(	\)	\ \	\|	\<

Limitations

• The * and + modifiers can only be applied to backslash expressions, not bare characters (e.g. \d+ is supported, 0+ is not)

• You cannot use alternation in a group, e.g. (foo|bar) is not permitted

• Complex backtracking is not supported, e.g. \p*\d*\s*\w*: does not match a single colon, because \p* consumes the colon

• . matches a literal dot, whereas \. matches any character

• \s matches only an ASCII space (32), not other whitespace like tab

• there is no syntax to match a literal caret, asterisk or plus (although \p will match asterisk or plus, along with some other characters)

Sregex (OS_Match) syntax

This is faster than OS_Regex, but only supports simple string matching and the following special characters.

Special characters

Expressions	Actions
^	To specify the beginning of the text
$	To specify the end of the text
|	To create a logic: or, between multiple patterns
!	To negate the expression

PCRE2 syntax

Perl Compatible Regular Expressions (PCRE) tries to match Perl syntax and semantics as closely as it can.

It provides features like recursive patterns, look-ahead and look-behind assertions, non-capturing groups, non-greedy quantifiers, extended syntax for characters and character classes, and many others. For more details, please refer to the PCRE Syntax documentation.

Supported expressions

Expressions	Actions
.	Any character except newline
\d	Any decimal digit, equal to [0-9]
\D	Any character that is not a decimal digit, equal to [^0-9]
\h	Any horizontal white space character
\H	Any character that is not a horizontal white space character
\s	Any white space character, equal to [\t\r\n\f]
\S	Any character that is not a white space character, equal to [^\t\r\n\f]
\w	Any “word” character
\W	Any “non-word” character

Characters escaping

Expressions	Actions
\f	Form feed (hex 0C)
\n	Newline (hex 0A)
\r	Carriage return (hex 0D)
\t	Tab (hex 09)
\0dd	Character with octal code 0dd
\o{ddd..}	Character with octal code ddd..
\xhh	Character with hex code hh
\x{hh..}	Character with hex code hh..

Quantifiers

Expressions	Actions
?	0 or 1, greedy
?+	0 or 1, possessive
??	0 or 1, lazy
*	0 or more, greedy
*+	0 or more, possessive
*?	0 or more, lazy
+	1 or more, greedy
++	1 or more, possessive
+?	1 or more, lazy
{n}	Exactly n
{n,m}	At least n, no more than m, greedy
{n,m}+	At least n, no more than m, possessive
{n,m}?	At least n, no more than m, lazy
{n,}	n or more, greedy
{n,}+	n or more, possessive
{n,}?	n or more, lazy