Backslash in Regex- PHP

Question

I am trying to learn Regex in PHP and stuck in here now. My ques may appear silly but pls do explain.
I went through a link:
Extra backslash needed in PHP regexp pattern

But I just could not understand something:

In the answer he mentions two statements:

2 backslashes are used for unescaping in a string ("\\\\" -> \\)
1 backslash is used for unescaping in the regex engine (\\ -> \)

My ques:
what does the word "unescaping" actually means? what is the purpose of unescaping? Why do we need 4 backslashes to include it in the regex?

Its called escaping not ? Escaping is required because some characters are defined as preseverd characters with their own function. To let PHP know u want the real character, not the preseverd one u need to escape it so PHP don't treat like a special one — DarkBee
– DarkBee, Commented Feb 17, 2014 at 11:57
Then two backslashes would do the job, why are we including 4 backslashes? — Abhinav
– Abhinav, Commented Feb 17, 2014 at 12:01

deceze · Accepted Answer · 2014-02-17 13:11:12Z

The backslash has a special meaning in both regexen and PHP. In both cases it is used as an escape character. For example, if you want to write a literal quote character inside a PHP string literal, this won't work:

$str = ''';

PHP would get "confused" which ' ends the string and which is part of the string. That's where \ comes in:

$str = '\'';

It escapes the special meaning of ', so instead of terminating the string literal, it is now just a normal character in the string. There are more escape sequences like \n as well.

This now means that \ is a special character with a special meaning. To escape this conundrum when you want to write a literal \, you'll have to escape literal backslashes as \\:

$str = '\\'; // string literal representing one backslash

This works the same in both PHP and regexen. If you want to write a literal backslash in a regex, you have to write /\\/. Now, since you're writing your regexen as PHP strings, you need to double escape them:

$regex = '/\\\\/';

One pair of \\ is first reduced to one \ by the PHP string escaping mechanism, so the actual regex is /\\/, which is a regex which means "one backslash".

Pateta · Accepted Answer · 2014-02-17 12:30:20Z

0

I think you can use "preg_quote()":

http://php.net/preg_quote

This function escapes special chars, so you can give an input as it is, without escaping by yourself:

<?php
$string = "online 24/7. Only for \o/";
$escaped_string = preg_quote($string, "/"); // 2nd param is optional and used if you want to escape also the delimiter of your regex
echo $escaped_string; // $escaped_string: "online 24\/7.  Only for \\o\/"
?>

answered Feb 17, 2014 at 12:30

Pateta

4294 silver badges16 bronze badges

Collectives™ on Stack Overflow

Backslash in Regex- PHP

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related