Look-around are non-capturing group that implements an assertion.
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 is typically used to create a logical operator AND between two regular expressions.
For example, if a password must:
then the following expression can be used to validate the password.
(?=.*[[:lower:]])(?=.*[[:upper:]])(?=.*[[:punct:]]).{6,}