Special characters: ^ $ \ . * + ? ( ) [ ] { } |

The characters ^ and $ are called anchors:

• ^ matches the beginning of the string
• $ matches the end of the string; the void after the last character

Parts of a regex can be repeated:

• * matches the preceding part zero or more times; * is equivalent to {0,}
• + matches the preceding part one or more times; + is equivalent to {1,}
• ? matches the preceding part zero or one time; ? is equivalent to {0,1}

{…} represents a bounded repeat:

• a{n} matches 'a' repeated exactly n times
• a{n,} matches 'a' repeated n or more times
• a{n,m} matches 'a' repeated between n and m times inclusive

Other simple patterns:

• . - any single character except the newline character
• \s - any whitespace character
• \S - any character that isn't a whitespace
• \b - a word boundary
• \B - any position that isn't a word boundary

The above repeats are greedy because they find the longest match. To make them non-greedy, add ? behind the repeat i.e., *?, +?, ??, {…}?.

Simple examples:

using System.Text.RegularExpressions;
...
// Check if all characters are numeric.
Regex reg = new Regex(@"^\d+$");
bool b1 = reg.Match("3451").Success; // true
bool b2 = reg.Match("34a1").Success; // false
 
// Check if a string has letters a,A,b,B.
bool b3 = Regex.IsMatch("acd", "a|b", RegexOptions.IgnoreCase); // true
bool b4 = Regex.IsMatch("eBd", "a|b", RegexOptions.IgnoreCase); // true
bool b5 = Regex.IsMatch("efg", "a|b", RegexOptions.IgnoreCase); // false
 
// Modify a string.
string s1 = Regex.Replace("aib", "a|b", "X"); // XiX

Example: Extract email addresses from a string:

public List<string> GetEmails(string str)
{
    const string Pattern = @"[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}";
    List<string> emails = new List<string>();
 
    Regex reg = new Regex(Pattern, RegexOptions.IgnoreCase);
    MatchCollection matches = reg.Matches(str);
    foreach (Match m in matches)
        emails.Add(m.Value);
 
    return emails;
}
...
string emailsStr = "leon@micro.com john@ Mel@kata.ca Phil@@lego.com";
List<string> emails = GetEmails(emailsStr); // emails = { "leon@micro.com", "Mel@kata.ca" }

Example: Use named groups to match patterns:

string str = @"Leon's GDI+ GDI++ +GDU Mike'ss Kale's Chloe''s";
Regex re = new Regex(@"((?<GroupPlusSign>\w+\+)|(?<GroupApostrophe>\w+'s))(?=(\s|$))", RegexOptions.None);
MatchCollection matches = re.Matches(str); // 3 matches: Leon's GDI+ Kale's
 
int i = 1;
foreach (Match m in matches)
{
    Console.WriteLine($"Match #{i}: " +
        $"GroupPlusSign={m.Groups["GroupPlusSign"].Value}, " +
        $"GroupApostrophe={m.Groups["GroupApostrophe"].Value}");
    ++i;
}

Output:

Match #1: GroupPlusSign=, GroupApostrophe=Leon's
Match #2: GroupPlusSign=GDI+, GroupApostrophe=
Match #3: GroupPlusSign=, GroupApostrophe=Kale's

Example: “hello out there, how are you”

Make * ungreedy:

Backreferences are used to capture matches for later reuse.

Example: “Today is monday the 18th.”

Named groups - .NET syntax:

string str = @"leon's GDI+ GDI++ +GDU Hello9's hi'ss ho''s";
Regex re = new Regex(@"((?<GroupPlusSign>\w+\+)|(?<GroupApostropheS>\w+'s))(?=(\s|$))", RegexOptions.None);
MatchCollection matches = re.Matches(sb.ToString());
foreach (Match m in matches)
{
    string word1 = m.Groups["GroupPlusSign"].Value; // matches GDI+
    string word2 = m.Groups["GroupApostropheS"].Value; // matches leon's and Hello9's
}

URL (IP):

^(http\://|https\://|ftp\://|)((([a-z_0-9\-]+)+(([\:]?)+([a-z_0-9\-]+))?)(\@+)?)?(((((([0-1])?([0-9])?[0-9])|(2[0-4][0-9])|(2[0-5][0-5])))\.(((([0-1])?([0-9])?[0-9])|(2[0-4][0-9])|(2[0-5][0-5])))\.(((([0-1])?([0-9])?[0-9])|(2[0-4][0-9])|(2[0-5][0-5])))\.(((([0-1])?([0-9])?[0-9])|(2[0-4][0-9])|(2[0-5][0-5]))))|((([a-z0-9\-])+\.)+([a-z]{2}\.[a-z]{2}|[a-z]{2,4})))(([\:])(([1-9]{1}[0-9]{1,3})|([1-5]{1}[0-9]{2,4})|(6[0-5]{2}[0-3][0-6])))?$

URL (port and IP allowed)

^(((ht|f)tp(s?))\://)?((([a-zA-Z0-9_\-]{2,}\.)+[a-zA-Z]{2,})|((?:(?:25[0-5]|2[0-4]\d|[01]\d\d|\d?\d)(?(\.?\d)\.)){4}))(:[a-zA-Z0-9]+)?(/[a-zA-Z0-9\-\._\?\,\'/\\\+&%\$#\=~]*)?$

Date:

^((((((0?[13578])|(1[02]))\-((0?[1-9])|([1-2][0-9])|(3[01])))|(((0?[469])|(11))\-((0?[1-9])|([1-2][0-9])|(30)))|(0?2\-((0?[1-9])|([1-2][0-9]))))\-\d{2}(([02468][048])|([13579][26])))|(((((0?[13578])|(1[02]))\-((0?[1-9])|([1-2][0-9])|(3[01])))|(((0?[469])|(11))\-((0?[1-9])|([1-2][0-9])|(30)))|(0?2\-((0?[1-9])|(1[0-9])|(2[0-8]))))\-\d{2}(([02468][1235679])|([13579][01345789]))))$

Time:

^(\s(((0?[1-9])|(1[0-9])|(2[0-3])|(0)|(00))\:([0-5][0-9])(|(\s)|(\:([0-5][0-9])))))?$

Regular expressions can be used for string-related operations such as:

• Validation: Check if an input string is well-formed.
• Parsing: Extract information from an input string.
• Transformation: Search substrings and replace them with a new substring.
• Iteration: Search all occurrences of a substring.
• Tokenization: Split a string into substrings.

• MSDN: Regular Expression Language - Quick Reference
• StackOverflow: Excluding certain patterns in a regex
• RegEx examples