Regular-Expression-in-Dot-Net-Framework-Part-III-Quantifiers-and-Backreference-Constructs

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.

Quantifiers

A quantifier specifies how many instances of the previous element (which can be a character, a group, or a character class) must be present in the input string for a match to occur. Quantifiers include the language elements listed in the following table.

Quantifier	Description	Pattern	Matches
*	Matches the previous element zero or more times.	\d*\.\d	".0", "19.9", "219.9"
+	Matches the previous element one or more times.	"be+"	"bee" in "been", "be" in "bent"
?	Matches the previous element zero or one time.	"rai?n"	"ran", "rain"
{ n }	Matches the previous element exactly n times.	",\d{3}"	",043" in "1,043.6", ",876", ",543", and ",210" in "9,876,543,210"
{ n ,}	Matches the previous element at least n times.	"\d{2,}"	"166", "29", "1930"
{ n , m }	Matches the previous element at least n times, but no more than m times.	"\d{3,5}"	"166", "17668" "19302" in "193024"
*?	Matches the previous element zero or more times, but as few times as possible.	\d*?\.\d	".0", "19.9", "219.9"
+?	Matches the previous element one or more times, but as few times as possible.	"be+?"	"be" in "been", "be" in "bent"
??	Matches the previous element zero or one time, but as few times as possible.	"rai??n"	"ran", "rain"
{ n }?	Matches the preceding element exactly n times.	",\d{3}?"	",043" in "1,043.6", ",876", ",543", and ",210" in "9,876,543,210"
{ n ,}?	Matches the previous element at least n times, but as few times as possible.	"\d{2,}?"	"166", "29", "1930"
{ n , m }?	Matches the previous element between n and m times, but as few times as possible.	"\d{3,5}?"	"166", "17668" "193", "024" in "193024"

Backreference Constructs

A backreference allows a previously matched subexpression to be identified subsequently in the same regular expression. The following table lists the backreference constructs supported by regular expressions in the .NET Framework.

Backreference construct	Description	Pattern	Matches
\ number	Backreference. Matches the value of a numbered subexpression.	(\w)\1	"ee" in "seek"
\k< name >	Named backreference. Matches the value of a named expression.	(?<char>\w)\k<char>	"ee" in "seek"