C# 4 and .NET 4

egular expressions ❘ 221
The following table lists some of the main special characters or escape sequences that you can use. It is not
comprehensive, but a fuller list is available in the MSDN documentation.
symbol meaning eXamPle maTChes
^ Beginning of input text ^B B, but only if fi rst character in text
$ End of input text X$ X, but only if last character in text
. Any single character except the
newline character (\ )
* Preceding character may be repeated
zero or more times
+ Preceding character may be repeated
one or more times
Preceding character may be repeated
zero or one time
i.ation
ra*t
ra+t
rat
isation, ization
rt, rat, raat, raaat, and so on
rat, raat, raaat and so on, but
not rt
rt and rat only
\s Any whitespace character \sa [space]a, \ta, \na (\t and \n
have the same meanings as in C#)
\S Any character that isn’t whitespace \SF aF, rF, cF, but not \tf
\b Word boundary ion\b Any word ending in ion
\B Any position that isn’t a word
boundary
\BX\B
Any X in the middle of a word
If you want to search for one of the meta - characters, you can do so by escaping the corresponding character
with a backslash. For example, . (a single period) means any single character other than the newline
character, whereas \. means a dot.
You can request a match that contains alternative characters by enclosing them in square brackets. For
example, [1|c] means one character that can be either 1 or c . If you wanted to search for any occurrence
of the words map or man , you would use the sequence ma[n|p] . Within the square brackets, you can also
indicate a range, for example [a - z] , to indicate any single lowercase letter, [A - E] to indicate any uppercase
letter between A and E (including the letters A and E themselves), or [0 – 9] to represent a single digit. If you
want to search for an integer (that is, a sequence that contains only the characters 0 through 9), you could
write [0 – 9]+ .
The use of the + character indicates there must be at least one such digit, but there may
be more than one — so this would match 9, 83, 854, and so on.
displaying results
In this section, you code the RegularExpressionsPlayaround example, so you can get a feel for how the
regular expressions work.
The core of the example is a method called WriteMatches() , which writes out all the matches from a
MatchCollection in a more detailed format. For each match, it displays the index of where the match was
found in the input string, the string of the match, and a slightly longer string, which consists of the match
plus up to ten surrounding characters from the input text — up to fi ve characters before the match and up to
fi ve a f t e r wa rd . ( I t i s f e we r t h a n fi ve c h a r ac t e r s i f t h e m at c h o c c u r r e d w it h i n fi ve c h a r ac t e r s of t h e b e g i n n i n g
or end of the input text.) In other words, a match on the word messaging that occurs near the end of the
input text quoted earlier would display and messaging of d (fi ve characters before and after the match),
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