Happy new year!! It has been way to quit on my blog lately. My new years resolution is to change this though. I have much in store, though I’m starting of with something small (but handy).

Simple replacement
String replacement is often used as a way to apply templating. You might replace “%a:test” with “~~test~~” using the regexp: %a:(\w+), replacing it with “~~$1~~”.

Trying to escape
The only problem now, is that I can’t use “%a:” any more within my string. This could be solved by allowing escaping using the backslash. In the regexp we can use a negative lookbehind to see if the character before the % isn’t a backslash: (?<!\\)%a:(\w+).

Escaping the escaping
Now we’re close, however now it’s not possible to use “\%:a” anywhere. We need to be able to escape the backslash as well. We could state the problem as needing to match %a if there isn’t an uneven number of backslashes in front of it. Checking for an uneven number in a negative lookbehind isn’t possible unfortunately, so we need to get the backslashes into the match. We can say: match 0 or more pairs of backslashes, followed by “%a:”, if there is no backslash in front of it. This results in the regexp:
(?<!\\)((?:\\{2})*+)%a:(\w+), replacing it for “$1~~$2~~”.

To finish up
To only thing is that \% and \\ will still be displayed as that. This can simply be solved with a str_replace.