Interpreting a regex

Question

I need help interpreting the meaning of a regular expression. I'm trying to understand to improve my skills with them...

The regular expression is:

`(?<!\\\\)u`

It is used in PHP with preg_replace.

Answer 1

(?<!PATTERN)

is a negative look-behind, where PATTERN must not be found before the match following the group, which is a \\ in this case. (I'm also going on the basis the the 4 \ is used due to escaping necessary within a php string)

See also this demo for a live example.

Answer 2

This means, "Find a u that is not preceded by \\." The ?<! construct is the negative lookbehind, and the \\ is an escaped \.