Negative Lookahead (?!)

Negative lookahead matches only when a pattern is NOT followed by something specific.

Syntax

pattern(?!lookahead) matches "pattern" only if NOT followed by "lookahead".

Matches numbers that are NOT followed by " dollars".

Exercises

Excluding Specific Patterns

Practical Examples

Combining with Positive Lookahead

You can use both positive and negative lookaheads:

Password Validation Example

A complex example combining concepts:

This password must:

• Have uppercase (?=.*[A-Z])
• Have lowercase (?=.*[a-z])
• Have a digit (?=.*\d)
• NOT contain "password" (?!.*password)
• Be at least 8 characters .{8,}

Practice Playground

Try:

• (?!test)\w+ - not starting with test
• \w+(?!_user) - not ending with _user
• (?!.*test).*admin.* - admin without test

Key Takeaways

• (?!pattern) is a negative lookahead
• Matches only when NOT followed by the pattern
• Zero-width: doesn't consume characters
• Useful for exclusion logic
• Combine with positive lookaheads for complex rules