C# 5.0 Programmer's Reference

Building Regular Expressions ❘ 475
Parentheses create groups. For example, consider the expression (\w)\1. The parentheses create a
numbered group that in this example matches a single word character. Later in the expression, the
text \1 refers to group number 1. That means this regular expression matches a word character followed
by itself. If the string is “book,” then this pattern would match the “oo” in the middle.
Group Indexes
In regular expressions, numbered groups are numbered starting at 1, not 0.
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 shown in the previous
example.
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.
To use a named group in a regular expression, use the syntax \k.
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. For example, the expression \d{3} matches any digit exactly 3 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 a B character followed by 1 or more Os, but it
matches as few Os as possible, so it would match only the BO in BOOK.
www.it-ebooks.info