Question

I am trying to parse a string with something like :

preg_match( "|from:(.*?);|", $string, $match);

But then I found that the string can also contain lfrom: and _from:

A few examples of how the string can be:

var1:34234;from:website1.com;lfrom:website2.com;var2:343423;
lfrom:website1.com;var1:4234234;from:website2.com
from:website1.com;_from:website2.com;lfrom:website2.com;var1:43523;

How can I parse only from:(.*?); and not lfrom, _from, etc.

Answer 1

I was gonna give you the solution but I better explain you about the lookbehind modifier.

In regex each time you “match” a h for example, that h will add 1 to the pointer of where the regex is at the moment so you dont want to “add” nothing to the pointer. You just want to look if the from is preceded by a ;\s\b or the start of the string. You don’t want to match the VOID because there are voids everywhere!!

So, an example: (?<a)b that would match a b that has an a before it. So it just does the next: When a b found it looks before it, if there is an a it matches the regex.

So… (?<=[;\s\b]|^)from:(\w+\.\w+) Would match a from that right before it has [;\s\b] OR ^ (The string start)

DEMO

Pretty easy, huh!?