php regex replace double backslash with single

Question

I do not want to use stripslashes() because I only want to replace "\\" with "\".

I tried preg_replace("/\\\\/", "\\", '2\\sin(\\pi s)\\Gamma(s)\\zeta(s) = i\\oint_C \\frac{-x^{s-1}}{e^x -1} \\mathrm{d}x');

Which to my disapointment returns: 2\\sin(\\pi s)\\Gamma(s)\\zeta(s) = i\\oint_C \\frac{-x^{s-1}}{e^x -1} \\mathrm{d}x

Various online regex testers indicate that the above should work. Why is it not?

what's your actual code? Are you doing $a = preg_replace(...$a) or are you only calling preg_replace without actually saving its result? — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Jan 12, 2014 at 15:46
First of all: Try preg_replace('/\\\\/', '\\', '2\\sin(...');(single quotes). Then read up on str_replace(), egexes are the wrong tool for the job. — Eugen Rieck
– Eugen Rieck, Commented Jan 12, 2014 at 15:46
A related question (replace single \ with double \): stackoverflow.com/questions/5631946/… — Uli Köhler
– Uli Köhler, Commented Jan 12, 2014 at 15:49

Stefan Nuxoll · Accepted Answer · 2014-01-12 15:51:23Z

11

First, like many other people are stating, regular expressions might be too heavy of a tool for the job, the solution you are using should work however.

$newstr = preg_replace('/\\\\/', '\\', $mystr);

Will give you the expected result, note that preg_replace returns a new string and does not modify the existing one in-place which may be what you are getting hung up on.

You can also use the less expensive str_replace in this case:

$newstr = str_replace('\\\\', '\\', $mystr);

This approach costs much less CPU time and memory since it doesn't need to compile a regular expression for a simple task such as this.

answered Jan 12, 2014 at 15:51

Stefan Nuxoll

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Uli Köhler · Accepted Answer · 2014-01-12 15:47:40Z

9

You dont need to use regex for this, use

$newstr = str_replace("\\\\", "\\", $mystr);

See the str_replace docs

answered Jan 12, 2014 at 15:47

Uli Köhler

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3 Comments

user2680766 Over a year ago

Your code will throw a parse error. It should be $newqsstr = str_replace("\\\\", "\\", $mystr);.

Uli Köhler Over a year ago

@SharanyaDutta Thanks, I must have miscopied the replace call. Edited that it. Sorry for the mistake!

Jonny 5 Over a year ago

@UliKöhler Simpler and better, right, because with regex "/\\\\/" you would match a single backslash, as the engine needs an escaped backslash to match a literal backslash, beause the backslash has a special meaning in regex. So it would be "/\\\\{2}/" to match a double backslash with a double-quoted str pattern.

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