Question

I used an online regular expression (regex) tester to build a simple regex, however using PHP’s preg_match it’s giving an unknown modifier for $.

Here is the regex:

if (preg_match('/(^Keyword1/$|^Keyword2/$)/', $input, $matches))

I’m trying to check if $input equals either Keyword1/ or Keyword2/ (exact match). I know I can easily do this with "if ($input == ‘Keyword1/’)", however I’d rather have a few lines of regex vs. a dozen if statements in the code.

How can I fix it?

Answer 1

You need to escape the / inside the regex, because / is also being used as delimiter:

if (preg_match('/(^Keyword1\/$|^Keyword2\/$)/', $input, $matches))
                           ^            ^

Alternatively, use a different delimiter:

if (preg_match('~(^Keyword1/$|^Keyword2/$)~', $input, $matches))
                ^                         ^

Since both your sub-regexs have common anchors, you can simplify your regex as:

 if (preg_match('~^(Keyword1|Keyword2)/$~', $input, $matches))

Why the warning in your regex?

'/(^Keyword1/$|^Keyword2/$)/'

Since you are using / as delimiter, the second / in your regex makes PHP think it is the end of your regex. Now PHP accepts regex modifiers like s, m, i after the closing delimiter. But in your case, PHP sees a $ after the closing delimiter. Since $ is not a valid modifier you get the warning:

PHP Warning: preg_match(): Unknown modifier ‘$’ in …