C# Regex Tester

Test your regular expressions against any string and see instant match highlighting. The generated C# code uses System.Text.RegularExpressions and is ready to paste into your project.

Regex Pattern

//g

Flags

gGlobal — find all matches (always on)iIgnore case → RegexOptions.IgnoreCasemMultiline → RegexOptions.MultilinesSingleline (dot matches \n) → RegexOptions.Singleline

Test String

Match Highlights

11 matches

Hello, World! This is a C# Regex Tester. Match Words that Start With a Capital Letter.

Match Details

"Hello"index 0, length 5

"World"index 7, length 5

"This"index 14, length 4

"Regex"index 27, length 5

"Tester"index 33, length 6

"Match"index 41, length 5

"Words"index 47, length 5

"Start"index 58, length 5

"With"index 64, length 4

"Capital"index 71, length 7

"Letter"index 79, length 6

Generated C# Code

using System.Text.RegularExpressions;

string pattern = @"\b[A-Z][a-z]+\b";
string input = "your text here";

// Check if the pattern matches
bool isMatch = Regex.IsMatch(input, pattern);

// Get all matches
MatchCollection matches = Regex.Matches(input, pattern);
foreach (Match match in matches)
{
    Console.WriteLine($"Match: '{match.Value}' at index {match.Index}");

    // Access named or numbered capture groups
    for (int i = 1; i < match.Groups.Count; i++)
    {
        Console.WriteLine($"  Group {i}: '{match.Groups[i].Value}'");
    }
}

// Replace matches
string result = Regex.Replace(input, pattern, "replacement");

How to Use Regex in C#

C# uses the System.Text.RegularExpressions namespace for regex support. The static Regex class provides methods for matching, replacing, and splitting strings without instantiating an object.

Common C# Regex Methods

Method	Returns	Description
Regex.IsMatch(input, pattern)	bool	True if the pattern matches anywhere in input
Regex.Match(input, pattern)	Match	First match object (check .Success before using)
Regex.Matches(input, pattern)	MatchCollection	All non-overlapping matches
Regex.Replace(input, pattern, replacement)	string	Replace each match with a replacement string
Regex.Split(input, pattern)	string[]	Split input at each regex match

C# RegexOptions (Flags)

Flag (this tester)	C# Option	Effect
i	RegexOptions.IgnoreCase	Case-insensitive matching
m	RegexOptions.Multiline	^ and $ match start/end of each line
s	RegexOptions.Singleline	. matches newline characters (\n)
—	RegexOptions.Compiled	Compiles to IL for repeated high-performance use
—	RegexOptions.IgnorePatternWhitespace	Allows whitespace and comments inside the pattern

Common Regex Patterns for C#

Pattern	What it matches
\d+	One or more digits (integer)
\d+\.\d+	Decimal number
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$	Email address (basic)
^https?://	URL starting with http:// or https://
\b\w{5}\b	Exactly 5-letter word
^\s*$	Blank or whitespace-only line
(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})	ISO date with named groups