wordpress: replacing square brackets in the_content

Answer 1

As far as I can tell there are a number of things wrong here. First, I would use preg_replace instead of preg_match. So I would write the function like this:

function test_function($content)
{
    $txt = '        
    <div id="\3_container" class="overlay">
        <img width="\1" height="\2" src="\3_thumb.jpg" />
    </div>';

    return preg_replace('%\[custom w="(\d+)" h="(\d+)"\]((?:[a-z][a-z][0-9]+[a-z0-9]))\[\/custom\]%', $txt, $content);
}

Which is basically 2 lines of code. Some other problems you're running into:

1) Your regex fails. The first [ isn't escaped, and as you know that's a special character in regexes. I suspect there are other problems with your regex too.

2) From the PHP manual for strpos:

This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE, such as 0 or "". Please read the section on Booleans for more information. Use the === operator for testing the return value of this function.

Answer 2

why don't you just use the WordPress shortcode api? -> http://codex.wordpress.org/Shortcode_API