C# 5.0 Programmer's Reference

838 ❘ Appendix T Regular Expressions
Grouping Constructs
Grouping constructs let you define capture groups within matching pieces of a string. Parentheses
create groups. There are several kinds of groups, some of which are fairly specialized and confusing.
The two most common are numbered and named groups.
To create a numbered group, simply enclose a subexpression in parentheses as in (\w)\1. The \w
in this expression matches a single word character. The parentheses mean this character is in the
first numbered group. The \1 that follows matches whatever is in group 1, in this case a single word
character. That means this expression matches a single word character that appears twice.
To create a named group, use the syntax (subexpression) where name is the name you
want to assign to the group and subexpression is a subexpression. Use the syntax \k to
refer to a named group.
For example, the expression (\w)\k is equivalent to the previous expression
(\w)\1 except the group is named twice.
Quantifiers
A quantifier makes the regular expression engine match the previous element a certain number of
times. The following table describes regular expression quantifiers.
Quantifier
Meaning
* Matches the previous element 0 or more times
+ Matches the previous element 1 or more times
Matches the previous element 0 or 1 times
{n}
{n,}
{n,m}
Matches the previous element exactly n times
Matches the previous element n or more times
Matches the previous element between n and m times (inclusive)
If you follow one of these with , the pattern matches the preceding expression as few times as possible.
For example, the pattern BO+ matches B followed by 1 or more Os, so it would match the BOO
in BOOK. The pattern BO+ also matches B followed by 1 or more Os, but it matches as few Os as possible,
so it would match only the BO in BOOK.
Alternation Constructs
An alternation construct uses the | character to allow a pattern to match either of two subexpressions.
For example, the expression ^(true|yes)$ matches either true or yes.
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