C# / .NET Regex Tester

Test regular expressions written for C# / .NET (System.Text.RegularExpressions) — with warnings for anything that works differently there, and C# / .NET code to copy.

//g

gglobaliinsensitivemmultilinesdotAll

Test String

C# / .NET regex syntax at a glance

Literal: new Regex(@"pattern") — a verbatim string, where " is written ""
Named group: (?<name>…) or (?'name'…), back-reference \k<name>
Replacement: $1, ${name}, $0
Flags: RegexOptions.IgnoreCase, Multiline, Singleline (dot matches \n), IgnorePatternWhitespace — or inline (?i)
Find all: foreach (Match m in regex.Matches(text))

What's different in C# / .NET

→\d matches any Unicode digit (Arabic-Indic, Devanagari…), not just 0–9 — use [0-9] or RegexOptions.ECMAScript when that matters.
→Lookbehind can be any length, and balancing groups (?<open-close>…) can match nested brackets.
→There are no possessive quantifiers; use an atomic group (?>…) instead.
→Pass a match timeout (new Regex(pattern, options, TimeSpan.FromSeconds(1))) for patterns that run on user input, and prefer [GeneratedRegex] in .NET 7+.

Frequently asked

Why does \d match characters that aren't 0–9 in C#?

.NET follows Unicode: \d is any decimal digit, including other scripts. Use [0-9], or RegexOptions.ECMAScript to get ASCII-only behavior.

What does RegexOptions.Singleline do?

It makes . match newline characters too (other languages call this dotAll or DOTALL). It doesn't change ^ and $ — that's RegexOptions.Multiline.