Regular expressions are powerful, but sometimes you need to match a pattern only if it's preceded or followed by another pattern. This is where lookarounds come in.
What are Lookarounds?
Lookarounds are zero-width assertions. They don't consume any characters in the string; they just assert whether a match is possible or not.
There are four types of lookarounds:
- Positive Lookahead
(?=...): Asserts that what immediately follows matches. - Negative Lookahead
(?!...): Asserts that what immediately follows does not match. - Positive Lookbehind
(?<=...): Asserts that what immediately precedes matches. - Negative Lookbehind
(?<!...): Asserts that what immediately precedes does not match.
Example: Positive Lookahead
Suppose you want to match a password that must contain at least one number. You can use a positive lookahead:
^(?=.*\d)[a-zA-Z\d]{8,}$
This pattern checks if there is a digit anywhere ahead in the string (?=.*\d), and then matches 8 or more alphanumeric characters.
Using Interactive Playgrounds
A great way to learn is by practicing. Try testing your lookarounds in our Playground to see how they match in real-time.
Lookarounds are supported in most modern regex engines, including JavaScript, Python, and PCRE. However, older browsers like Safari had delayed support for lookbehinds.
Keep experimenting with lookarounds to unlock the full potential of regular expressions!