php - PCRE regular expression overlapping matches

Question

i have the following string

001110000100001100001

and this expression

/[1]....[1]/g

this makes two matches

but i want it to also match the pattern between those both with lookbehind so to say, the overlapping 1

i have absolutely no clue, how can this work ? instead of 0 it can be any characters

Answer 1

A common trick is to use capturing technique inside an unanchored positive lookahead. Use this regex with preg_match_all:

(?=(1....1))

See regex demo

The values are in $matches[1]:

$re = "/(?=(1....1))/"; 
$str = "001110000100001100001"; 
preg_match_all($re, $str, $matches);
print_r($matches[1]);

See lookahead reference:

Lookaround actually matches characters, but then gives up the match, returning only the result: match or no match. That is why they are called "assertions". They do not consume characters in the string, but only assert whether a match is possible or not.

If you want to store the match of the regex inside a lookahead, you have to put capturing parentheses around the regex inside the lookahead, like this: (?=(regex)).