What is a regex lookahead?

A lookahead is a zero-width assertion that checks whether a pattern follows the current position without consuming any characters. (?=pattern) is a positive lookahead (must be followed by pattern); (?!pattern) is a negative lookahead (must NOT be followed by pattern).

What is a regex lookbehind?

A lookbehind checks the characters before the current position without consuming them. (?<=pattern) is a positive lookbehind (must be preceded by pattern); (?<!pattern) is a negative lookbehind (must NOT be preceded by pattern).

Why are lookarounds called zero-width?

Because they match a position rather than consuming characters. A lookahead checks what comes next, but the matched text does not include the lookahead content — only what the main pattern captures.

Does JavaScript support lookbehind?

Yes, since ES2018. Both positive (?<=...) and negative (?<!...) lookbehinds are supported in all modern browsers and Node.js 10+. The FixTools tester supports them since it runs in the browser.

Can I use variable-length lookbehinds in JavaScript?

Yes. Unlike PCRE (which requires fixed-width lookbehinds), JavaScript supports variable-length lookbehinds since ES2018. Patterns like (?<=a+) are valid in JS.

What is the difference between a lookahead and a non-capturing group?

A non-capturing group (?:...) is part of the match and consumes characters. A lookahead (?=...) only checks a condition at the current position and consumes nothing — the matched text excludes the lookahead content.

Browse tools

Free · Fast · Privacy-first

Regex Lookahead and Lookbehind Tester

Q: Why are lookarounds called zero-width?

Because they match a position rather than consuming characters. A lookahead checks what comes next, but the matched text does not include the lookahead content — only what the main pattern captures.

Q: Does JavaScript support lookbehind?

Yes, since ES2018. Both positive (?<=...) and negative (?<!...) lookbehinds are supported in all modern browsers and Node.js 10+. The FixTools tester supports them since it runs in the browser.

Q: Can I use variable-length lookbehinds in JavaScript?

Yes. Unlike PCRE (which requires fixed-width lookbehinds), JavaScript supports variable-length lookbehinds since ES2018. Patterns like (?<=a+) are valid in JS.

Q: What is the difference between a lookahead and a non-capturing group?

A non-capturing group (?:...) is part of the match and consumes characters. A lookahead (?=...) only checks a condition at the current position and consumes nothing — the matched text excludes the lookahead content.

Lookahead and lookbehind are some of the most powerful — and misunderstood — features in regex. They let you match content based on surrounding context without consuming those surrounding characters. FixTools lets you test all four lookaround types with live highlighting so you can see exactly what is and is not matched.

⚡ Use Tool Now How it works

Cost: Free forever
Sign-up: Not required
Processing: In your browser
Privacy: Files stay local

⚡

Test positive and negative lookaheads live

🔒

Test positive and negative lookbehinds live

✨

Zero-width assertions shown in match highlights

Developer Tool

Regex Tester

All processing happens in your browser — your files are never uploaded to any server.

🚀Open Regex Tester

100% Free · No account · Works on any device

How to use this tool

💡

Try a positive lookahead: \d+(?= dollars) matches the number in "100 dollars" but not in "100 euros". Paste this pattern and test string to see how lookahead works in practice.

How It Works

Step-by-step guide to regex lookahead and lookbehind tester:

1
Understand the four lookaround types
(?=...) positive lookahead: match if followed by pattern. (?!...) negative lookahead: match if NOT followed by pattern. (?<=...) positive lookbehind: match if preceded by pattern. (?<!...) negative lookbehind: match if NOT preceded by pattern.
2
Test a positive lookahead
Pattern: \w+(?=\s+Street) — test string: "Baker Street, Elm Avenue, Oak Street". Matches "Baker" and "Oak" but not "Elm".
3
Test a negative lookahead
Pattern: \d+(?! dollars) — test string: "100 euros, 50 dollars, 200 pounds". Matches "100" (followed by euros) and "200" (followed by pounds) but not "50" (followed by dollars).
4
Test a lookbehind
Pattern: (?<=\$)\d+ — test string: "$100 and £200". Matches "100" (preceded by $) but not "200" (preceded by £).
5
Combine lookarounds
Lookaheads and lookbehinds can be combined: (?<=\$)\d+(?=\.\d{2}) matches the integer part of a dollar amount only when followed by decimal cents.