Regex Tester – Real-Time Regular Expression Testing with Match Highlighting

Regex Tester

A real-time regex testing tool with match highlighting, capture groups, and named groups: devnestio.pages.dev/regex-tester/

Features

• Real-time matching — results update as you type (80ms debounce)
• Color-coded highlights — up to 5 colors cycle through matches
• Capture groups listed — numbered ($1, $2) and named groups
• Match positions — start and end index for every match
• Flag toggle pills — click g/i/m/s/u/d to toggle flags instantly
• Regex error display — clear error messages for invalid patterns
• Quick reference cheat sheet — \d, \w, \b, lookahead, named groups, etc.
• Pre-loaded with email regex example

Supported Flags

Flag	Meaning
g	Global — find all matches
i	Case insensitive
m	Multiline — ^ and $ match line boundaries
s	DotAll — . matches newlines too
u	Unicode mode
d	Has indices — adds start/end index to groups

Zero-Width Match Protection

Zero-width matches (like \b or (?=x)) can cause infinite loops with global flag. The tool handles this:

while ((m = re.exec(testStr)) !== null) {
  matches.push(m);
  if (m[0].length === 0) re.lastIndex++; // advance past zero-width
}

Named Groups Example

Pattern:  (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
String:   2026-07-01
Groups:   year=2026  month=07  day=01

Try It

devnestio.pages.dev/regex-tester/

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Part of the DevNestio developer tools collection.