About
Look-around are non-capturing group that implements an assertion.
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List
If you don't find your pattern below, question mark has other functionality in regular expression. See them all at Regular Expression - Question Mark (?)
| Assertion | |
|---|---|
| (?=X) | X, positive lookahead (via zero-width) |
| (?!X) | X, negative lookahead (via zero-width) |
| (?<=X) | X, positive lookbehind (via zero-width) |
| (?<!X) | X, negative lookbehind (via zero-width) |
where X is a sub-regular expression
Lookahead
Lookahead is typically used to create a logical operator AND between two regular expressions.
For example, if a password must:
- contain:
- a lower case letter,
- an upper case letter,
- a punctuation symbol,
- and be at least 6 characters long
then the following expression can be used to validate the password.
(?=.*[[:lower:]])(?=.*[[:upper:]])(?=.*[[:punct:]]).{6,}