simple regex isn't working using preg_replace_callback()

Question

I'm trying to use preg_replace_callback to fill in variables in an imported document (which I control), based on this answer, but it's not working. As far as I can tell, the callback is never invoked, which would mean the regex is never being matched.

The bare-bones content of my doc.html file:

<p>test {$test} $test test</p>

The PHP:

$test = "ham";
$allVars = get_defined_vars();

$filename = "/path/to/doc.html";
$html = file_get_contents($filename);
$html = preg_replace_callback("/\$[a-zA-Z_][a-zA-Z0-9_]*/", "find_replacements", $html);

echo($html);
exit();

// replace callback function
function find_replacements($match) {
    global $allVars;
    if (array_key_exists($match[0], $allVars))
        return $allVars[$match[0]];
    else
        return $match[0];
}

The output is <p>test {$test} $test test</p>, but I was expecting <p>test {ham} ham test</p>

I wouldn't use $ as PHP already uses it for interpolation, it could lead to nasty bugs. Try using a different character, like test {#test} #test test — elclanrs
– elclanrs, Commented Jan 21, 2014 at 19:26
Also try putting your regex in single quotes if you use $. — elclanrs
– elclanrs, Commented Jan 21, 2014 at 19:30
That was it -- using single quotes around my regex. Can you write up an answer explaining why? — Blazemonger
– Blazemonger, Commented Jan 21, 2014 at 19:32
Nasty bug! Answer below is good, but my solution would be to just chose a different character. — elclanrs
– elclanrs, Commented Jan 21, 2014 at 19:33

Jeff Lambert · Accepted Answer · 2014-01-21 19:32:37Z

1

First, the dollar sign in regex is being interpolated by PHP because the regex is in double quotes. Put single quotes around that:

$html = preg_replace_callback('/\$[a-zA-Z_][a-zA-Z0-9_]*/', "find_replacements", $html);

Second, the values sent to your callback include the dollar sign, whereas the dollar sign is not present in the $allVars array, so you must manually strip it off:

function find_replacements($match) {
    global $allVars;
    $match[0] = substr($match[0],1);
    if (array_key_exists($match[0], $allVars))
        return $allVars[$match[0]];
    else
        return $match[0];
}

Making those modifications, I was able to receive this output:

test {ham} ham test

answered Jan 21, 2014 at 19:32

Jeff Lambert

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1 Comment

Blazemonger Over a year ago

I spotted that second problem after I submitted the question, and fixed by using $allVars[$match[1]] and adding parentheses in my regex: '/\$([a-zA-Z_][a-zA-Z0-9_]*)/' -- Thanks for an alternate solution, though!

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