PHP Regex throwing error messages

Question

I have looked at the documentation that PHP provides on PCRE Patterns. I am using a third party plugin to handle some text from the user, and the following preg_replace is failing because of missing terminating ] char. (preg_replace(): Compilation failed: missing terminating ] for character class

$input  = preg_replace('/[\]/i','',$userInput);

From what I can see the terminating delimiter is / with a character class that only has a \ in it. The i, if I can read correctly tells the expression to not care about upper or lower case. I see both opening [ and closing ].

Why is it throwing the error? What is the preg_replace trying to do?

To match a backslash use \\\\ !

HamZa
– HamZa

2013-03-29 19:01:10 +00:00
Commented Mar 29, 2013 at 19:01 — HamZa
– HamZa, Commented Mar 29, 2013 at 19:01
@Ham: \\\ is what you want.

webbiedave
– webbiedave

2013-03-29 19:04:13 +00:00
Commented Mar 29, 2013 at 19:04 — webbiedave
– webbiedave, Commented Mar 29, 2013 at 19:04
@webbiedave Ah yes, that will also work :P

HamZa
– HamZa

2013-03-29 19:05:16 +00:00
Commented Mar 29, 2013 at 19:05 — HamZa
– HamZa, Commented Mar 29, 2013 at 19:05

Michael · Accepted Answer · 2013-03-29 19:02:05Z

3

You need to escape the \ otherwise it escapes the ] (and you need to escape it twice, once for the PHP string and once for PCRE).

$input  = preg_replace('/[\\\]/i','',$userInput);

And you can omit the [ and ] altogether (as well as the i).

$input  = preg_replace('/\\\/','',$userInput);

Or, you can just use str_replace:

$input  = str_replace('\\','',$userInput);

answered Mar 29, 2013 at 19:02

Michael

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3 Comments

Kris.Mitchell Over a year ago

So you are telling me that whole thing was to replace \\ with blanks. I don't know why the script would do that, but re-write it I shall. Thanks!

Michael Over a year ago

I'm assuming it's a very poor attempt at escaping for an SQL query, but not know what library you're using I can only speculate.

MikeM Over a year ago

As a rule it is better to use four backslashes to match a backslash in PHP. Although it does not apply in this case, if another backslashed character was following the three backslashes, the pattern may not be interpreted as intended.

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