Positive Lookbehind

Lookbehinds check what comes BEFORE a position. Positive lookbehind matches when preceded by a specific pattern.

Syntax

(?<=lookbehind)pattern matches "pattern" only if preceded by "lookbehind".

Matches numbers only when preceded by "$" - but "$" isn't part of the match!

Exercises

Lookbehind vs Regular Match

Compare these approaches:

With lookbehind - Only the value is matched:

Without lookbehind - Everything is matched:

Lookbehind is cleaner when you only want the value.

Practical Examples

Combining with Lookahead

Use both for precise matching:

Practice Playground

Try:

• (?<=@)\w+ - after @
• (?<=\$)[\d.]+ - after $
• (?<=#)\w+ - after #
• (?<=: ).+ - after ": "

Lookbehind Limitations

Lookbehinds must have a fixed length in most regex engines:

• ✅ (?<=abc) - fixed 3 characters
• ✅ (?<=ab|cd) - fixed alternatives
• ❌ (?<=a+) - variable length (not allowed in many engines)

JavaScript supports variable-length lookbehinds, but not all languages do.

Key Takeaways

• (?<=pattern) is a positive lookbehind
• Matches only when preceded by the pattern
• Zero-width: doesn't consume characters
• Cleaner than capturing groups for extraction
• Combine with lookahead for precise matching