Performance of tokenizing CSS in PHP
- by Boldewyn
This is a noob question from someone who hasn't written a parser/lexer ever before.
I'm writing a tokenizer/parser for CSS in PHP (please don't repeat with 'OMG, why in PHP?'). The syntax is written down by the W3C neatly here (CSS2.1) and here (CSS3, draft).
It's a list of 21 possible tokens, that all (but two) cannot be represented as static strings.
My current approach is to loop through an array containing the 21 patterns over and over again, do an if (preg_match()) and reduce the source string match by match. In principle this works really good. However, for a 1000 lines CSS string this takes something between 2 and 8 seconds, which is too much for my project.
Now I'm banging my head how other parsers tokenize and parse CSS in fractions of seconds. OK, C is always faster than PHP, but nonetheless, are there any obvious D'Oh! s that I fell into?
I made some optimizations, like checking for '@', '#' or '"' as the first char of the remaining string and applying only the relevant regexp then, but this hadn't brought any great performance boosts.
My code (snippet) so far:
$TOKENS = array(
'IDENT' => '...regexp...',
'ATKEYWORD' => '@...regexp...',
'String' => '"...regexp..."|\'...regexp...\'',
//...
);
$string = '...CSS source string...';
$stream = array();
// we reduce $string token by token
while ($string != '') {
$string = ltrim($string, " \t\r\n\f"); // unconsumed whitespace at the
// start is insignificant but doing a trim reduces exec time by 25%
$matches = array();
// loop through all possible tokens
foreach ($TOKENS as $t => $p) {
// The '&' is used as delimiter, because it isn't used anywhere in
// the token regexps
if (preg_match('&^'.$p.'&Su', $string, $matches)) {
$stream[] = array($t, $matches[0]);
$string = substr($string, strlen($matches[0]));
// Yay! We found one that matches!
continue 2;
}
}
// if we come here, we have a syntax error and handle it somehow
}
// result: an array $stream consisting of arrays with
// 0 => type of token
// 1 => token content
