Regex Lookaround
Assert context without consuming: lookahead and lookbehind.
- lookahead
- lookbehind
- password regex
- zero-width
- regex assert
Regex Lookaround
Assert context without consuming: lookahead and lookbehind.
Commands
| Command | What it does |
|---|---|
foo(?=bar) | foo followed by bar |
foo(?!bar) | foo NOT followed by bar |
(?<=@)\w+ | Word after at-sign |
^(?=.*[A-Z])(?=.*\d).{8,}$ | Password rule via lookaheads |
Example
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[A-Za-z\d@$!%*?&]{8,}$FAQ
Why is my lookbehind rejected in JavaScript?
Older engines allow only lookahead; modern V8 supports fixed-length lookbehind - keep it fixed-width.
Lookaround vs consuming patterns?
Lookaround asserts context and keeps the cursor in place, so overlapping conditions stay checkable.
الأسئلة الشائعة
Why is my lookbehind rejected in JavaScript?
Older engines allow only lookahead; modern V8 supports fixed-length lookbehind - keep it fixed-width.
Lookaround vs consuming patterns?
Lookaround asserts context and keeps the cursor in place, so overlapping conditions stay checkable.