A regular expression is a pattern that could be matched against an input text. The .Net framework provides a regular expression engine that allows such matching. A pattern consists of one or more character literals, operators, or constructs.

Constructs for Defining Regular Expressions

There are various categories of characters, operators, and constructs that lets you to define regular expressions. Click the following links to find these constructs.
  • Character escapes
These are basically the special characters or escape characters. The backslash character (\) in a regular expression indicates that the character that follows it either is a special character or should be interpreted literally. The following table lists the escape characters −
Escape character Description Pattern Matches
\a Matches a bell character, \u0007. \a "\u0007" in "Warning!" + '\u0007'
\b In a character class, matches a backspace, \u0008. [\b]{3,} "\b\b\b\b" in "\b\b\b\b"
\t Matches a tab, \u0009. (\w+)\t "Name\t", "Addr\t" in "Name\tAddr\t"
\r Matches a carriage return, \u000D. (\r is not equivalent to the newline character, \n.) \r\n(\w+) "\r\nHello" in "\r\Hello\nWorld."
\v Matches a vertical tab, \u000B. [\v]{2,} "\v\v\v" in "\v\v\v"
\f Matches a form feed, \u000C. [\f]{2,} "\f\f\f" in "\f\f\f"
\n Matches a new line, \u000A. \r\n(\w+) "\r\nHello" in "\r\Hello\nWorld."
\e Matches an escape, \u001B. \e "\x001B" in "\x001B"
\nnn Uses octal representation to specify a character (nnn consists of up to three digits). \w\040\w "a b", "c d" in "a bc d"
\x nn Uses hexadecimal representation to specify a character (nn consists of exactly two digits). \w\x20\w "a b", "c d" in "a bc d"
\c X\c x Matches the ASCII control character that is specified by X or x, where X or x is the letter of the control character. \cC "\x0003" in "\x0003" (Ctrl-C)
\u nnnn Matches a Unicode character by using hexadecimal representation (exactly four digits, as represented by nnnn). \w\u0020\w "a b", "c d" in "a bc d"
\ When followed by a character that is not recognized as an escaped character, matches that character. \d+[\+-x\*]\d+\d+[\+-x\*\d+ "2+2" and "3*9" in "(2+2) * 3*9"
  • Character classes
A character class matches any one of a set of characters. The following table describes the character classes −
Character class Description Pattern Matches
[character_group] Matches any single character in character_group. By default, the match is case-sensitive. [mn] "m" in "mat" "m", "n" in "moon"
[^character_group] Negation: Matches any single character that is not in character_group. By default, characters incharacter_group are case-sensitive. [^aei] "v", "l" in "avail"
[ first - last ] Character range: Matches any single character in the range from first to last. [b-d] [b-d]irds Birds Cirds Dirds
. Wildcard: Matches any single character except \n. a.e "ave" in "have" "ate" in "mate"
\p{ name } Matches any single character in the Unicode general category or named block specified by name. \p{Lu} "C", "L" in "City Lights"
\P{ name } Matches any single character that is not in the Unicode general category or named block specified by name. \P{Lu} "i", "t", "y" in "City"
\w Matches any word character. \w "R", "o", "m" and "1" in "Room#1"
\W Matches any non-word character. \W "#" in "Room#1"
\s Matches any white-space character. \w\s "D " in "ID A1.3"
\S Matches any non-white-space character. \s\S " _" in "int __ctr"
\d Matches any decimal digit. \d "4" in "4 = IV"
\D Matches any character other than a decimal digit. \D " ", "=", " ", "I", "V" in "4 = IV"
  • Anchors
Anchors allow a match to succeed or fail depending on the current position in the string. The following table lists the anchors −
Assertion Description Pattern Matches
^ The match must start at the beginning of the string or line. ^\d{3} "567" in "567-777-"
$ The match must occur at the end of the string or before \nat the end of the line or string. -\d{4}$ "-2012" in "8-12-2012"
\A The match must occur at the start of the string. \A\w{3} "Code" in "Code-007-"
\Z The match must occur at the end of the string or before \n at the end of the string. -\d{3}\Z "-007" in "Bond-901-007"
\z The match must occur at the end of the string. -\d{3}\z "-333" in "-901-333"
\G The match must occur at the point where the previous match ended. \\G\(\d\) "(1)", "(3)", "(5)" in "(1)(3)(5)[7](9)"
\b The match must occur on a boundary between a \w(alphanumeric) and a \W(nonalphanumeric) character. \w "R", "o", "m" and "1" in "Room#1"
\B The match must not occur on a \b boundary. \Bend\w*\b "ends", "ender" in "end sends endure lender"
  • Grouping constructs
Grouping constructs delineate sub-expressions of a regular expression and capture substrings of an input string. The following table lists the grouping constructs −
Grouping construct Description Pattern Matches
( subexpression ) Captures the matched subexpression and assigns it a zero-based ordinal number. (\w)\1 "ee" in "deep"
(?< name >subexpression) Captures the matched subexpression into a named group. (?< double>\w)\k< double> "ee" in "deep"
(?< name1 -name2 >subexpression) Defines a balancing group definition. (((?'Open'\()[^\(\)]*)+((?'Close-Open'\))[^\(\)]*)+)*(?(Open)(?!))$ "((1-3)*(3-1))" in "3+2^((1-3)*(3-1))"
(?: subexpression) Defines a noncapturing group. Write(?:Line)? "WriteLine" in "Console.WriteLine()"
(?imnsx-imnsx:subexpression) Applies or disables the specified options within subexpression. A\d{2}(?i:\w+)\b "A12xl", "A12XL" in "A12xl A12XL a12xl"
(?= subexpression) Zero-width positive lookahead assertion. \w+(?=\.) "is", "ran", and "out" in "He is. The dog ran. The sun is out."
(?! subexpression) Zero-width negative lookahead assertion. \b(?!un)\w+\b "sure", "used" in "unsure sure unity used"
(?< =subexpression) Zero-width positive lookbehind assertion. (?< =19)\d{2}\b "99", "50", "05" in "1851 1999 1950 1905 2003"
(?< ! subexpression) Zero-width negative lookbehind assertion. (?< !19)\d{2}\b "51", "03" in "1851 1999 1950 1905 2003"
(?> subexpression) Nonbacktracking (or "greedy") subexpression. [13579](?>A+B+) "1ABB", "3ABB", and "5AB" in "1ABB 3ABBC 5AB 5AC"
  • Quantifiers
Quantifiers specify 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.
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
Backreference constructs allow a previously matched sub-expression to be identified subsequently in the same regular expression. The following table lists these constructs −
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"
  • Alternation constructs
Alternation constructs modify a regular expression to enable either/or matching. The following table lists the alternation constructs −
Alternation construct Description Pattern Matches
| Matches any one element separated by the vertical bar (|) character. th(e|is|at) "the", "this" in "this is the day. "
(?( expression )yes | no ) Matches yes if expression matches; otherwise, matches the optional no part. Expression is interpreted as a zero-width assertion. (?(A)A\d{2}\b|\b\d{3}\b) "A10", "910" in "A10 C103 910"
(?( name )yes | no ) Matches yes if the named capture name has a match; otherwise, matches the optional no. (?< quoted>")?(?(quoted).+?"|\S+\s) Dogs.jpg, "Yiska playing.jpg" in "Dogs.jpg "Yiska playing.jpg""
  • Substitutions
Substitutions are used in replacement patterns. The following table lists the substitutions −
Character Description Pattern Replacement pattern Input string Resulting string
$number Substitutes the substring matched by group number. \b(\w+)(\s)(\w+)\b $3$2$1 "one two" "two one"
${name} Substitutes the substring matched by the named groupname. \b(?< word1>\w+)(\s)(?< word2>\w+)\b ${word2} ${word1} "one two" "two one"
$$ Substitutes a literal "$". \b(\d+)\s?USD $$$1 "103 USD" "$103"
$& Substitutes a copy of the whole match. (\$*(\d*(\.+\d+)?){1}) **$& "$1.30" "**$1.30**"
$` Substitutes all the text of the input string before the match. B+ $` "AABBCC" "AAAACC"
$' Substitutes all the text of the input string after the match. B+ $' "AABBCC" "AACCCC"
$+ Substitutes the last group that was captured. B+(C+) $+ "AABBCCDD" AACCDD
$_ Substitutes the entire input string. B+ $_ "AABBCC" "AAAABBCCCC"
  • Miscellaneous constructs
The following table lists various miscellaneous constructs −
Construct Definition Example
(?imnsx-imnsx) Sets or disables options such as case insensitivity in the middle of a pattern. \bA(?i)b\w+\b matches "ABA", "Able" in "ABA Able Act"
(?#comment) Inline comment. The comment ends at the first closing parenthesis. \bA(?#Matches words starting with A)\w+\b
# [to end of line] X-mode comment. The comment starts at an unescaped # and continues to the end of the line. (?x)\bA\w+\b#Matches words starting with A

The Regex Class

The Regex class is used for representing a regular expression. It has the following commonly used methods −
Sr.No. Methods & Description
1 public bool IsMatch(string input) Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.
2 public bool IsMatch(string input, int startat) Indicates whether the regular expression specified in the Regex constructor finds a match in the specified input string, beginning at the specified starting position in the string.
3 public static bool IsMatch(string input, string pattern) Indicates whether the specified regular expression finds a match in the specified input string.
4 public MatchCollection Matches(string input) Searches the specified input string for all occurrences of a regular expression.
5 public string Replace(string input, string replacement) In a specified input string, replaces all strings that match a regular expression pattern with a specified replacement string.
6 public string[] Split(string input) Splits an input string into an array of substrings at the positions defined by a regular expression pattern specified in the Regex constructor.
For the complete list of methods and properties, please read the Microsoft documentation on C#.

Example 1

The following example matches words that start with 'S' − 
using System;
using System.Text.RegularExpressions;

namespace RegExApplication {
   class Program {
      private static void showMatch(string text, string expr) {
         Console.WriteLine("The Expression: " + expr);
         MatchCollection mc = Regex.Matches(text, expr);
         
         foreach (Match m in mc) {
            Console.WriteLine(m);
         }
      }
      static void Main(string[] args) {
         string str = "A Thousand Splendid Suns";
         
         Console.WriteLine("Matching words that start with 'S': ");
         showMatch(str, @"\bS\S*");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result − 
Matching words that start with 'S':
The Expression: \bS\S*
Splendid
Suns

Example 2

The following example matches words that start with 'm' and ends with 'e' − 
using System;
using System.Text.RegularExpressions;

namespace RegExApplication {
   class Program {
      private static void showMatch(string text, string expr) {
         Console.WriteLine("The Expression: " + expr);
         MatchCollection mc = Regex.Matches(text, expr);
         
         foreach (Match m in mc) {
            Console.WriteLine(m);
         }
      }
      static void Main(string[] args) {
         string str = "make maze and manage to measure it";

         Console.WriteLine("Matching words start with 'm' and ends with 'e':");
         showMatch(str, @"\bm\S*e\b");
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result − 
Matching words start with 'm' and ends with 'e':
The Expression: \bm\S*e\b
make
maze
manage
measure

Example 3

This example replaces extra white space − 
using System;
using System.Text.RegularExpressions;

namespace RegExApplication {
   class Program {
      static void Main(string[] args) {
         string input = "Hello   World   ";
         string pattern = "\\s+";
         string replacement = " ";
         
         Regex rgx = new Regex(pattern);
         string result = rgx.Replace(input, replacement);

         Console.WriteLine("Original String: {0}", input);
         Console.WriteLine("Replacement String: {0}", result);    
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result − 
Original String: Hello World   
Replacement String: Hello World