Regular expression negative lookbehind

March 7, 2024

I want to have a matching regexp pattern that matches all the addresses that end in 4 or more digits, but not coming after 'APT', 'BOX', 'APT ', or 'BOX '.
So it should match these cases

HITME 1234
HITME 12345
HITME1234

but not the following cases

BOX 1234
BOX 12345
BOX4044
APT 1234
APT 12345
NONHIT123
NONHIT 123

I have made this one

(?<!(APT |BOX ))([0-9]{4,})$

but it does not work right. Somehow still matches the no no cases.

>Solution :

TL;DR use ^(?!APT|BOX).*?([0-9]{4,})$

Your regex (?<!(APT |BOX ))([0-9]{4,})$ incorrectly matches:

BOX 12345 on 2345 because it is not preceded by APT or BOX . Instead, it is preceded by BOX 1
BOX4044 on 4044 because it is not preceded by APT or BOX . Instead, it is preceded by BOX
APT 12345 on 2345 for a similar reason.

The regex you’re looking for is ^(?!APT|BOX).*?([0-9]{4,})$, which is broken down like so:

^(?!APT|BOX) – the beginning of the string cannot be followed by APT or BOX
.*? – a bunch of garbage in the middle of the string, taking as few characters as possible (i.e. HITME in your test cases)
([0-9]{4,})$ – the matched digits at the end of the string