Appendix A. Regular Expressions

The following tables summarize the regular-expression grammar and syntax supported by the regular-expression classes in System.Text.RegularExpression. Each of the modifiers and qualifiers in the tables can substantially change the behavior of the matching and searching patterns. For further information on regular expressions, we recommend the definitive Mastering Regular Expressions by Jeffrey E. F. Friedl (O'Reilly & Associates, 1997).

All the syntax described in the tables should match the Perl5 syntax, with specific exceptions noted.

Table A-1. Character escapes
Escape code sequence	Meaning	Hexadecimal equivalent
`\a`	Bell	`\u0007`
`\b`	Backspace	`\u0008`
`\t`	Tab	`\u0009`
`\r`	Carriage return	`\u000A`
`\v`	Vertical tab	`\u000B`
`\f`	Form feed	`\u000C`
`\n`	Newline	`\u000D`
`\e`	Escape	`\u001B`
`\040`	ASCII character as octal
`\x20`	ASCII character as hex
`\cC`	ASCII control character
`\u0020`	Unicode character as hex
`\non-escape`	A nonescape character

Special case: within a regular expression, \b means word boundary, except in a [ ] set, in which \b means the backspace character.

Table A-2. Substitutions
Expression	Meaning
`$group-number`	Substitutes last substring matched by group-number
`${group-name}`	Substitutes last substring matched by (`?<group-name>`)

Substitutions are specified only within a replacement pattern.

Table A-3. Character sets
Expression	Meaning
.	Matches any character except \n
`[characterlist]`	Matches a single character in the list
`[^characterlist]`	Matches a single character not in the list
`[char0-char1]`	Matches a single character in a range
`\w`	Matches a word character; same as `[a-zA-Z_0-9]`
`\W`	Matches a nonword character
`\s`	Matches a space character; same as `[\n\r\t\f]`
`\S`	Matches a nonspace character
`\d`	Matches a decimal digit; same as `[0-9]`
`\D`	Matches a nondigit

Table A-4. Positioning assertions
Expression	Meaning
`^`	Beginning of line
`$`	End of line
`\A`	Beginning of string
`\Z`	End of line or string
`\z`	Exactly the end of string
`\G`	Where search started
`\b`	On a word boundary
`\B`	Not on a word boundary

Table A-5. Quantifiers
Quantifier	Meaning
`*`	0 or more matches
`+`	1 or more matches
`?`	0 or 1 matches
`{n`}	Exactly n matches
`{n`,}	At least n matches
`{n`,`m`}	At least n, but no more than m matches
`*?`	Lazy `*`, finds first match that has minimum repeats
`+?`	Lazy `+`, minimum repeats, but at least 1
`??`	Lazy `?`, zero or minimum repeats
`{n`}?	Lazy {n}, exactly n matches
`{n`,}?	Lazy {n}, minimum repeats, but at least n
`{n`,`m`}?	Lazy {n,m}, minimum repeats, but at least n, and no more than m

Table A-6. Grouping constructs
Syntax	Meaning
`( )`	Capture matched substring
`(?<name`>)	Capture matched substring into group name^[1]
`(?<number`>)	Capture matched substring into group number^*
`(?<name1`-`name2`>)	Undefine name2, and store interval and current group into name1; if name2 is undefined, matching backtracks; name1 is optional^*
`(?: )`	Noncapturing group
`(?imnsx-imnsx: )`	Apply or disable matching options
`(?= )`	Continue matching only if subexpression matches on right
`(?! )`	Continue matching only if subexpression doesn't match on right
`(?<= )`	Continue matching only if subexpression matches on left
`(?<! )`	Continue matching only if subexpression doesn't match on left
`(?> )`	Subexpression is matched once, but isn't backtracked

^[1] Single quotes may be used instead of angle bracketsfor example (?'name').

The named capturing group syntax follows a suggestion made by Friedl in Mastering Regular Expressions. All other grouping constructs use the Perl5 syntax.

Table A-7. Back references
Parameter syntax	Meaning
`\count`	Back reference count occurrences
`\k<name`>	Named back reference

Table A-8. Alternation
Expression syntax	Meaning
`\|`	Logical OR
`(?(expression`)yes\|no)	Matches `yes` if expression matches, else `no`; the `no` is optional
`(?(name`)yes\|no)	Matches `yes` if named string has a match, else `no`; the `no` is optional

Table A-9. Miscellaneous Constructs
Expression syntax	Meaning
`(?imnsx-imnsx)`	Set or disable options in midpattern
`(?# )`	Inline comment
# [to end of line]	X-mode comment

Table A-10. Regular expression options
Option	Meaning
`i`	Case-insensitive match
`m`	Multiline mode; changes `^` and `$` so they match beginning and ending of any line
`n`	Capture explicitly named or numbered groups
`c`	Compile to MSIL
`s`	Single-line mode; changes meaning of "." so it matches every character
`x`	Eliminates unescaped whitespace from the pattern
`r`	Search from right to left; can't be specified in midstream