Regular Expression in Dot Net Framework Part IV - Alternation Constructs and Substitutions

A regular expression is a pattern that the regular expression engine attempts to match in input text. A pattern consists of one or more character literals, operators, or constructs.

Alternation Constructs

Alternation constructs modify a regular expression to enable either/or matching. These constructs include the language elements listed in the following table.

Alternation construct	Description	Pattern	Matches
|	Matches any one element separated by the vertical bar (|) character.	th(e|is|at)	"the", "this" in "this is the day. "
(?( expression ) yes | no )	Matches yes if the regular expression pattern designated by expression matches; otherwise, matches the optional no part. expression is interpreted as a zero-width assertion.	(?(A)A\d{2}\b|\b\d{3}\b)	"A10", "910" in "A10 C103 910"
(?( name ) yes | no )	Matches yes if name, a named or numbered capturing group, has a match; otherwise, matches the optional no.	(?<quoted>")?(?(quoted).+?"|\S+\s)	Dogs.jpg, "Yiska playing.jpg" in "Dogs.jpg "Yiska playing.jpg""

Substitutions

Substitutions are regular expression language elements that are supported in replacement patterns.

Character	Description	Pattern	Replacement pattern	Input string	Result string
$ number	Substitutes the substring matched by group number.	\b(\w+)(\s)(\w+)\b	$3$2$1	"one two"	"two one"
${ name }	Substitutes the substring matched by the named group name.	\b(?<word1>\w+)(\s)(?<word2>\w+)\b	${word2} ${word1}	"one two"	"two one"
$$	Substitutes a literal "$".	\b(\d+)\s?USD	$$$1	"103 USD"	"$103"
$&	Substitutes a copy of the whole match.	(\$*(\d*(\.+\d+)?){1})	**$&	"$1.30"	"**$1.30**"
$`	Substitutes all the text of the input string before the match.	B+	$`	"AABBCC"	"AAAACC"
$'	Substitutes all the text of the input string after the match.	B+	$'	"AABBCC"	"AACCCC"
$+	Substitutes the last group that was captured.	B+(C+)	$+	"AABBCCDD"	AACCDD
$_	Substitutes the entire input string.	B+	$_	"AABBCC"	"AAAABBCCCC"