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Regex Lookahead and Lookbehind Guía rápida

Use lookaround assertions to match context without consuming characters in the final match.

What this cheat sheet is for

This regex lookahead and lookbehind reference helps you complete common developer tasks quickly and avoid repeated trial-and-error when working under deadlines.

  • When you need a reliable command or pattern during debugging and incident response.
  • When you are implementing a feature and want a known-good syntax example immediately.
  • When onboarding teammates and documenting repeatable workflows for daily use.
  • When reviewing pull requests and verifying that command usage follows best practices.
  • Match password rules without capturing everything.
  • Replace numbers only when followed by units.
  • Extract tokens with boundary context conditions.

Positive and negative lookahead

Require suffix without consuming it

/foo(?=bar)/

Example:

/\d+(?=px)/

Exclude suffix patterns

/foo(?!bar)/

Example:

/\b\w+(?!\.tmp)\b/

Positive and negative lookbehind

Require prefix before match

/(?<=\$)\d+(?:\.\d{2})?/

Example:

/(?<=ID-)\d+/

Exclude prefix patterns

/(?

Example:

/(?

Combined assertions

Password rule example

/^(?=.*[A-Z])(?=.*\d).{8,}$/

Example:

/^(?=.*[a-z])(?=.*[!@#$%^&*]).{10,}$/

Token extraction with boundaries

/(?<=token=)[A-Za-z0-9_-]+/

Example:

/(?

Common mistakes / Pitfalls

  • Lookbehind is unsupported in some older runtimes, especially legacy browsers.
  • Nested lookarounds can hurt readability and performance in large texts.
  • Zero-width assertions do not consume text, so global matching may seem surprising.

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Last updated: February 2026