CODEWITHSUNDEEP
phpregex
Hugo H
Hugo H

Reputation: 6362

preg_match to select match in first selector

I have this string to test and can't manage to make it works as I want :

<?php
$string = '500 or fatal error ?';
$regex = '#(fatal|500)#';
if (preg_match($regex, $string, $return)) {
    // Return 500
    echo $return[1];
}
?>

I want my script to return fatal instead of 500 because it's my first selector. Is it possible ?

Thanks by advance, and sorry for my poor english.

P.S. : Maybe my title isn't really clear, but I don't know how to explain it better. If you find it, change it.

Upvotes: 3

Views: 451

Answers (3)

Anirudha
Anirudha

Reputation: 32807

There are two kinds of regular expression engines: text-directed engines, and regex-directed engines.

php is regex-directed engine which would always return the leftmost match, even if "500" match could be found later

The engine will start at the first character of the string. It will try all possible permutations of the regular expression at the first character. Only if all possibilities have been tried and found to fail, will the engine continue with the second character in the text. Again, it will try all possible permutations of the regex, in exactly the same order. The result is that the regex-directed engine will return the leftmost match.

You can find more info here

Upvotes: 2

nickb
nickb

Reputation: 59709

If you want to do it with only one regex, you can do it like this:

$selectors = array( 'fatal', '500'); // Array of selectors (in priority order)

$delimiter = '/';
$regex = $delimiter . '(' . implode('|', array_map( 'preg_quote', $selectors, array_fill( 0, count( $selectors), '/'))) . ')' . $delimiter;
preg_match_all( $regex, $string, $matches);

See which of the selectors we found, in order of the array:

$found_selectors = array_flip( $matches[1]);
foreach( $selectors as $selector) { 
    if( isset( $found_selectors[$selector])) 
        return $selector;
}

Note that I flipped the $matches[1] array to get constant lookup time into the array, instead of doing an array_search() / in_array() which will both do a linear search.

You can see from this demo that this produces fatal, given your input string.

Upvotes: 2

just somebody
just somebody

Reputation: 19257

it's not possible as you are envisioning, but a different implementation will make it work

$string = '500 or fatal error';
$regexes = array('#\bfatal\b#', '#\b500\b#');

$match = null;
foreach ($regexes as $re)
  if (preg_match($re, $string, $match))
    break;
# $match is null -> not found

Upvotes: 2

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