Backreference does not work in PHP

Question

Lately I\'ve been studying (more in practice to tell the truth) regex, and I\'m noticing his power. This demand made by me (link), I am aware of \'backreference\'. I think I

Answer 1

It is because you use a double quoted string, inside a double quoted string \1 is read as the octal notation of a character (the control character SOH = start of heading), not as an escaped 1.

So two ways:

use single quoted string:

'/\[(b|i|u|s)\]\s*(.*?)\s*\[\/\1\]/i'

or escape the backslash to obtain a literal backslash (for the string, not for the pattern):

"/\[(b|i|u|s)\]\s*(.*?)\s*\[\/\\1\]/i"

As an aside, you can write your pattern like this:

$pattern = '~\[([bius])]\s*(.*?)\s*\[/\1]~i';

// with oniguruma notation
$pattern = '~\[([bius])]\s*(.*?)\s*\[/\g{1}]~i';

// oniguruma too but relative:
// (the second group on the left from the current position)
$pattern = '~\[([bius])]\s*(.*?)\s*\[/\g{-2}]~i';