The Backslash Problem in PHP

The backslash is a special character in PHP. This means that in a string, if we want the slash to represent itself, we must double it:

<?php
	$str = '\\ \\\\ \\\\\\'; // actually the string is '\ \\ \\\'
?>

The backslash is also a special character in regex. This means that inside a regex, for the slash to represent itself, it needs to be written as many as four times:

<?php
	echo preg_replace('#\\\\#', '!', '\\ \\\\ \\\\\\'); // will output '! ! !!'
?>

Let's now write a regex where the search pattern is: backslash one or more times. In this case, we will write the operator + for as many as 4 characters before it without grouping:

<?php
	echo preg_replace('#\\\\+#', '!', '\\ \\\\ \\\\\\'); // will output '! ! !'
?>

Given a string:

<?php
	$str = 'a\\b c\\d e\\f';
?>

Find all substrings in this string matching the pattern letter, backslash, letter.

Given a string:

<?php
	$str = 'a\\b c\\\\d e\\\\\\f';
?>

Find all substrings in this string matching the pattern letter, backslash any number of times, letter.