Today I learned:
Regex Lookahead and Lookbehind
(?=foo) |
Lookahead | Asserts that what immediately follows the current position in the string is foo |
(?<=foo) |
Lookbehind | Asserts that what immediately precedes the current position in the string is foo |
(?!foo) |
Negative Lookahead | Asserts that what immediately follows the current position in the string is not foo |
(?<!foo) |
Negative Lookbehind | Asserts that what immediately precedes the current position in the string is not foo |
Example
Expression: Hell(?=o) - Lookahead
HellishfailsHellopassesMelloHellopasses
Use
The basic example above isn’t very helpful. But it can be very useful for password rule validation or advanced find/replaces.
