PHP preg_replace not working as intended

Question

I am trying to replace /admin and \admin from the following two strings:

F:\dev\htdocs\cms\admin
http://localhost/cms/admin

Using the following regular expression in preg_replace:

/[\/\\][a-zA-Z0-9_-]*$/i

1) From the first string it just replaces admin where as it should replace \admin

2) From the second string it replaces every thing except http: where as it should replace only /admin

I have checked this expression on http://regexpal.com/ and it works perfect there but not in PHP.

Any idea?

Note that the last part of each string admin is not fixed, it can be any user selected value and thats why I have used [a-zA-Z0-9_-]* in regular expression.

In the first bracket, i require either a \ or / but only one at a time — asim-ishaq
– asim-ishaq, Commented Dec 28, 2013 at 16:54

Amal · Accepted Answer · 2013-12-28 17:34:30Z

6

The original regular expression should be /[\/\\][a-zA-Z0-9_-]*$/i, but since you need to escape the backslashes in string declarations as well, each backslash must be expressed with \\ -- 4 backslashes in total.

From the PHP manual:

Single and double quoted PHP strings have special meaning of backslash. Thus if \ has to be matched with a regular expression \\, then "\\\\" or '\\\\' must be used in PHP code.

So, your preg_replace() statement should look like:

echo preg_replace('/[\/\\\\][a-zA-Z0-9_-]*$/i', '', $str);

Your regex can be improved as follows:

echo preg_replace('~[/\\\\][\w-]*$~', '', $str);

Here, \w matches the ASCII characters [A-Za-z0-9_]. You can also avoid having to escape the forward slash / by using a different delimiter -- I've used ~ above.

edited Dec 28, 2013 at 17:34

answered Dec 28, 2013 at 17:07

Amal

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4 Comments

Jonny 5 Over a year ago

Imho that's only right for patterns in double-quotes. Using single-quotes 3 backslashes should be fine, why not? E.g. echo preg_replace('/(\\\)([\\\])/', '\1_\2_', "\\\\");

Glavić Over a year ago

@AmalMurali: since you used \w escape sequence, you don't need /i modifier anymore.

Amal Over a year ago

@Jonny5: That works, too, but requires changing the OP's original regex. Since \] is an invalid escape sequence, it will remain as it is, and that's probably why it works (although I'm not too sure about it).

Jonny 5 Over a year ago

Assuming you have a string \test, and want to replace \t : "~\\\t~" will NOT work, whereas "~\\\\t~" and '~\\\t~' will work. Imho the escape level for escaping a backslash when using single-quoted pattern is 1 level less.

revo · Accepted Answer · 2013-12-28 17:10:00Z

0

[\/\\\][a-zA-Z0-9_-]*$/i

Live demo

answered Dec 28, 2013 at 17:10

revo

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