Appendix B. Regular Expressions

The following tables summarize the regular expression grammar and syntax supported by the regular expression classes in System.Text.RegularExpressions. Each of the modifiers and qualifiers in the tables can substantially change the behavior of the matching and searching patterns. For further information on regular expressions, we recommend the definitive Mastering Regular Expressions by Jeffrey E. F. Friedl (O’Reilly).

All the syntax described in Table B-1 through Table B-10 should match the Perl5 syntax, with specific exceptions noted.

Table B-1. Character escapes

Escape Code Sequence

Meaning

Hexadecimal equivalent

a

Bell

u0007


Backspace

u0008

Tab

u0009
Carriage return 
u000D
v
Vertical tab
u000B
f
Form feed
u000C
Newline
u000A
e
Escape
u001B
40
ASCII character as octal
x20
ASCII character as hex
cC
ASCII control character
u0020
Unicode character as hex
non-escape
A nonescape character
Special case: within a regular expression,  means word boundary, except in a [] set, in which  means the backspace character.
Table B-2. Substitutions
Expression
Meaning
$group-number
Substitutes last substring matched by
            group-number 
${group-name}
Substitutes last substring matched by (?<group-name>) 
$$
Substitutes a literal “$”
$&
Substitutes copy of the entire match
$'
Substitutes text of the input string preceding
            match
$'
Substitutes text of the input string following
            match
$+
Substitutes the last captures group
$_
Substitutes the entire input string
Substitutions are specified only within a replacement
    pattern.
Table B-3. Character sets
 Expression
Meaning
.
Matches any character except
            n
[characterlist]
Matches a single character in the list
[^characterlist]
Matches a single character not in the
            list
[char0-char1]
Matches a single character in a range
w
Matches a word character; same as [a-zA-Z_0-9]
W
Matches a nonword character
s
Matches a space character; same as [ 

	vf]
S
Matches a nonspace character
d
Matches a decimal digit; same as [0-9]
D
Matches a nondigit
Table B-4. Positioning assertions
Expression
Meaning
^
Beginning of line
$
End of line
A
Beginning of string

End of line or string
z
Exactly the end of string
G
Where search started

On a word boundary
B
Not on a word boundary
Table B-5. Quantifiers
 Quantifier
Meaning
*
0 or more matches
+
1 or more matches
?
0 or 1 matches
{n}
Exactly n
            matches
{n,}
At least
            nmatches
{n,m}
At least n, but no more
            than m matches 
*?
Lazy *, finds
            first match that has minimum repeats
+?
Lazy +, minimum
            repeats, but at least 1
??
Lazy ?, zero or
            minimum repeats
{n}?
Lazy {n}, exactly
            n matches 
{n,}?
Lazy {n}, minimum repeats,
            but at least n 
{n,m}? 
Lazy {n,m}, minimum
            repeats, but at least n, and no more
            than m 
Table B-6. Grouping constructs
Syntax
Meaning
( )
Capture matched substring
(?<name>)
Capture matched substring into group
            name[a] 
(?<number>)
Capture matched substring into group
            number[a] 
(?<name1-name2>)
Undefine name2 and store
            interval and current group into name1;
            if name2 is undefined, matching
            backtracks; name1 is
            optional[a] 
(?: )
Noncapturing group
(?imnsx-imnsx: )
Apply or disable matching options
(?= )
Continue matching only if subexpression matches on
            right[c] 
(?! )
Continue matching only if subexpression doesn’t match
            on right[c] 
(?<= )
Continue matching only if subexpression matches on
            left[b][c] 
(?<! )
Continue matching only if subexpression doesn’t match
            on left[b][c] 
(?> )
Subexpression is matched once, but isn’t
            backtracked
[a] Single quotes may be used instead of angle brackets, for
                example (?'name').
[b] This construct doesn’t backtrack; this is to remain
                compatible with Perl5.
[c]  Zero-width assertion; does
                not consume any characters.
Tip
The named capturing group syntax follows a suggestion made by
      Jeffrey Friedl in Mastering Regular Expressions
      (O’Reilly). All other grouping constructs use the Perl5 syntax.
Table B-7. Back references
Parameter syntax
Meaning
count
Back reference count
            occurrences
k<name> 
Named back reference
Table B-8. Alternation
Expression syntax
Meaning
|
Logical OR
(?(expression)yes|no)
Matches yes if
            expression matches, else no;
            the no is optional
            
(?(name)yes|no)
Matches yes if
            named string has a match, else no; the no is optional 
Table B-9. Miscellaneous constructs
Expression syntax
Meaning
(?imnsx-imnsx)
Set or disable options in midpattern
(?# )
Inline comment
# [to
            end of line]
X-mode comment (requires x option or IgnorePatternWhitespace) 
Table B-10. Regular expression options
Option
RegexOption value
Meaning
i
IgnoreCase
Case-insensitive match
m
MultiLine
Multiline mode; changes ^ and $ so they match beginning and ending of
            any line 
n
ExplicitCapture
Capture explicitly named or numbered
            groups
Compiled
Compile to MSIL
s
SingleLine
Single-line mode; changes meaning of "." so it matches every character
            
x
IgnorePatternWhitespace
Eliminates unescaped whitespace from the
            pattern
RightToLeft
Search from right to left; can’t be specified in
            midstream 

		

		  






						
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