how do u escape "$" in reg expressions?

Anon

Hi,

Does anyone know if it is possible to escape a dollar sign so that it can be used as a literal in a regular expression? For example ...

this works to find a pattern matching "?abc":

$regEx = "^(\?abc)$";
$str = "?abc";
ereg($regEx, $str, $arr );
echo($arr[0]);

but an attempt to find "$abc" using a similar method does not work:

$regEx = "^(\$abc)$";
$str = "$abc";
ereg($regEx, $str, $arr );
echo($arr[0]);

I know the dollar is reserved as an end of pattern identifier in regular expressions but why can't it be escaped, am I missing something?

Oli

lawnmowerman

I know it doesn't appear that this should work, but it does. Try this:

$regEx = "^(\$abc)$";

-Rich

Anon

Hi Rich,

Thanks for replying. The servers I'm using return the following error when I use the double backslash so:

$regEx = "^(\$abc)$";
$str = "$abc";
ereg($regEx, $str, $arr );
echo($arr[0]);

generates:

Warning: REG_EPAREN: in c:\inetpub\wwwroot\oli\testregex.php on line 6

... where line 6 is the call to ereg(). Any thoughts?

Oli

lawnmowerman

Hi.

Keep in mind that when you have a '$' within quotes there are 2 behaviors:

1) Single quotes: A '$' is interpretted as a string contant '$'

2) Double quotes: A '$' is interpretted as the first character in a variable name. This behavior can be negated by escaping the '$' within double quotes.

If you change your first to lines from:

$regEx = "^(\$abc)$";
$str = "$abc";

to:

$regEx = '^(\$abc)$';
$str = '$abc';

all will work.

The problem is that, when using double quotes and not escaping the '$', variable substitution is taking place, making your strings:

$regEx = "^()$"
and
$str = ""

Sorry if I mislead you with my first reply....my mistake.

-Rich

Anon

Cheers Rich,

That clears it up. I got confused because I was thinking the problem was just with ereg() interpreting the dollar as an end of pattern identifier. It didn't occur to me that PHP would treat "$x" as a variable in this case because ever since I started coding in PHP I adopted the convention of closing quotes and concatenating when inserting variables into strings eg.

$var = "this";
$str = "like" . $var . "example";

I do this because it preserves the colour-coding of the variable name within the text editor (just dropping the variable name into the string without closing the quotes causes the variable name to be greyed out with the rest of the string). Hence, I had forgotten that this was even allowable. Not that you need to know any of this but I thought I'd try and excuse my ineptitude 😉

Thanks again,

Oli

lawnmowerman

Hey, we all fall into little problems cuz we're in another frame of mind 🙂

I, too, use the string concatenation method, but usually just for arrays ($x[test]) and objects ($this->set(1)) and the like.

-Rich

Volitics

Hello;

I've got the eregi to escape the dollar sign using double escape marks like this:

(eregi ("^{([[:alnum:]]|\$|[[:space:]]){0,10}$",} stripslashes($Price)))

When I tried to use single quote marks and one backslash before the dollar sign I got error messages.

Is it alright to use double quote marks and double backslashes?

FatalError

Volitics wrote:
Is it alright to use double quote marks and double backslashes?

Um...wouldn't that make a backslash and then a dollar sign? You'd get '\$', because '\' means you're escaping the '\', not the '$'.

Volitics

I'm not sure but it looks like the first backslash turns the dollar sign into a variable then the second backslash turns it into a literal.

The ereg page in the PHP Manual does not say much about escaping dollar signs:
http://us2.php.net/manual/en/function.ereg.php

About half way down the page a user's comment (by Jason Smart) says this:

09-Sep-2005 11:01
Tries to escape with \ in a bracket expression
You cannot with ereg functions (preg you can) so
ereg("^([-a-zA-Z0-9_\.\!@#\$&\*\+\=\|])*$" , $var)
should be
<?php ereg("^([-a-zA-Z0-9_.!@#$&*+=|])*$", $var); ?>

I tried the example from Jason Smart's posting and it seems to work:

<?php $var = "$";
if (ereg("^([-a-zA-Z0-9_.!@#$&*+=|])*$", $var)){
	echo "it seems to work";
	}?>

The manual does not say anything about backslashing the dollar sign. Or if it does I can't find it. And an unorthodox example like above seems to work ok.

Neither way looks like it ought to work but both work.

Darnest thing I ever saw.

My only question is if somebody tries to call the script on a Mac (my desktop is a Windows Me version) or a newer version of Windows will it give them an error message?

FatalError

Ah, that's because it's in square brackets in the above code. You don't need to escape dollar signs if they are in square brackets, but outside of them you do.