Regex Tester Pro

Overview

Test a regex against a body of text with full match metadata: capture groups, named groups, replacement preview, match positions, and a side panel showing the cheat-sheet entry for each token in your pattern. Targets the .NET regex engine but supports the common flavours' shared subset.

It's for developers building log parsers, input validators, search-and-replace operations, or rewriting URLs - anywhere regex is the right tool and getting it right matters. Reach for it when iterating on a tricky pattern, validating a replacement before pasting it into code, or learning what each metacharacter in someone else's regex does.

How it works

The tester compiles your pattern with the .NET System.Text.RegularExpressions engine - a backtracking NFA with RegexOptions flags (IgnoreCase, Multiline, Singleline, IgnorePatternWhitespace). .NET's regex syntax is well-documented in Microsoft's regex reference and is largely compatible with PCRE for typical patterns.

Capture groups are displayed in order; named groups ((?<name>...)) are extracted by name. Replacement previews use the standard $1, ${name}, $& substitution syntax. Common presets (URL, email, IPv4) are available so you can compare your custom pattern against a known-good baseline.

Examples

• Capture date parts:

  Pattern: (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
  Input:   "2026-05-18"
  Groups:  year=2026, month=05, day=18
  

• Replacement:

  Pattern:     (\w+)\s+(\w+)
  Replacement: $2 $1
  Input:       "Alex Morgan"
  Output:      "Morgan Alex"
  

• Multiline match:

  Pattern:  ^Error.*$
  Flags:    Multiline
  -> matches each error line independently
  

• IgnoreCase:

  Pattern: foo
  Flags:   IgnoreCase
  Input:   "FOO Bar foo"
  -> two matches
  

FAQ

Is the .NET engine compatible with my JavaScript regex?

For the common subset, yes. Differences: .NET supports balancing groups and variable-width lookbehind that JS doesn't; JS supports sticky y flag that .NET doesn't.

What flags are supported?

IgnoreCase (i), Multiline (m), Singleline (s), IgnorePatternWhitespace (x), ExplicitCapture, RightToLeft. Hover any flag for a tooltip.

Why doesn't my regex match?

Common pitfalls: anchors (^/$) match line boundaries only with Multiline; \b requires word characters on one side; lookbehind requires the regex engine to support it. Check the cheat-sheet panel for token-level help.

How fast is it?

.NET regex is JIT-compiled on first use. Patterns with catastrophic backtracking still hang - the engine has a timeout safety net to prevent runaway evaluation.

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