php preg_replace returns 0 value when using underscore character

Question

My question is, why does the position of the '_' (underscore) character cause this problem?

I have inherited a script that is using php's preg_replace in a function. The regex used is returning a 0 on any number it is used on.

function foo($number){ 
  $number = preg_replace('/[a-z$,-_]/i','',$number);
  // more code...
}

I did a bunch of debugging and found the problem was with the preg_replace(). A co-worker mentioned that the order of the characters in the regex maybe causing the bug. So, I played with this and found it to be true. The position of the '_' (underscore) character is the sinister culprit. I changed this to:

'/[a-z$_,-]/i'

... and everything works fine.

So, the question, again, is why does the position of the '_' (underscore) character cause this problem? I've Googled on this but have not found it and I thought the minds in this forum may have the answer.

Thanks for any enlightenment! -jc

I suspect it's more the position of the - (hyphen) character than the underscore. — CD001
– CD001, Commented May 28, 2014 at 13:40
@Deadooshka trifle overzealous but yeah, that would work - you only really need to escape the hyphen in this example '/[a-z$,\-_]/i' since the regexp itself is in single-quotes no interpolation will occur. — CD001
– CD001, Commented May 28, 2014 at 13:47
so it is always safe to precede a non-alphanumeric with "\" to specify that it stands for itself. — Deadooshka
– Deadooshka, Commented May 28, 2014 at 13:58

Wiseguy · Accepted Answer · 2014-05-28 13:40:44Z

3

It's the position of the hyphen, not the underscore. With [a-z$,-_], you're inadvertently creating a character range from , to _. Put the hyphen on the end or escape it.

Comma , is ASCII 0x2C, underscore _ is 0x5F, and digits fall between those (0x30 to 0x39).
(ref: ASCII table)

answered May 28, 2014 at 13:40

Wiseguy

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1 Comment

John Cowan Over a year ago

Thank you! That makes sense and I'm kicking myself for not seeing that in the reading I've done on ranges in regexs. I should have realized that even if the text wasn't explicit. Cheers!

tylerlindell · Accepted Answer · 2014-05-28 13:49:11Z

0

Some characters need to be escaped like this

[a-z,\-_]

edited May 28, 2014 at 13:49

answered May 28, 2014 at 13:40

tylerlindell

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3 Comments

CD001 Over a year ago

however, underscore is not one of those characters, the hyphen within the square brackets on the other hand...

John Cowan Over a year ago

escaping the hyphen does the trick. I tested this in my script. Both answers were spot on.

John Cowan Over a year ago

Escaping the hyphen will be more intuitive to a future coder who may inherit this script. I've commented both solutions but who knows if the comment will remain. Thank you all!

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