Positive Lookahead (?=)

Lookaheads check what comes AFTER a position without including it in the match. They're "zero-width assertions" - they don't consume characters.

Syntax

pattern(?=lookahead) matches "pattern" only if followed by "lookahead".

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

Understanding Zero-Width

The key concept: lookaheads check without consuming.

Only the first "foo" matches because only it's followed by "bar". Notice "bar" isn't highlighted - it's not part of the match.

Exercises

Multiple Conditions

You can chain lookaheads for multiple conditions:

This checks that the string contains both uppercase AND digits.

Practical Examples

Lookahead at Word Boundaries

Practice Playground

Try:

• \d+(?= dollars) - amounts in dollars
• \w+(?= \d) - words before numbers
• (?=.*dollars).* - lines containing "dollars"

Key Takeaways

• (?=pattern) is a positive lookahead
• It checks what follows WITHOUT including it in the match
• Zero-width: doesn't consume characters
• Can chain multiple lookaheads for AND logic
• Useful for conditional matching