PHP Preg-Replace more than one underscore

Question

How do I, using preg_replace, replace more than one underscore with just one underscore?

Answer 1

The + operator (quantifier) matches multiple instances of the last character (, character class or capture group or back-reference).

$string = preg_replace('/_+/', '_', $string);

This would replace one or more underscores with a single underscore.

---

Technically more correct to the title of the question then is to only replace two or more:

$string = preg_replace('/__+/', '_', $string);

Or writing the quantifier with braces:

$string = preg_replace('/_{2,}/', '_', $string);

And perhaps then to capture and (back-) reference:

$string = preg_replace('/(_)\1+/', '\1', $string);

Answer 2

preg_replace('/[_]+/', '_', $your_string);

Answer 3

Actually using /__+/ or /_{2,}/ would be better than /_+/ since a single underscore does not need to be replaced. This will improve the speed of the preg variant.

Answer 4

Running tests, I found this:

while (strpos($str, '__') !== false) {
    $str = str_replace('__', '_', $str);
}

to be consistently faster than this:

$str = preg_replace('/[_]+/', '_', $str);

I generated the test strings of varying lengths with this:

$chars = array_merge(array_fill(0, 50, '_'), range('a', 'z'));
$str = '';
for ($i = 0; $i < $len; $i++) {  // $len varied from 10 to 1000000
    $str .= $chars[array_rand($chars)];
}
file_put_contents('test_str.txt', $str);

and tested with these scripts (run separately, but on identical strings for each value of $len):

$str = file_get_contents('test_str.txt');
$start = microtime(true);
$str = preg_replace('/[_]+/', '_', $str);
echo microtime(true) - $start;

and:

$str = file_get_contents('test_str.txt');
$start = microtime(true);
while (strpos($str, '__') !== false) {
    $str = str_replace('__', '_', $str);
}
echo microtime(true) - $start;

For shorter strings the str_replace() method was as much as 25% faster than the preg_replace() method. The longer the string, the less the difference, but str_replace() was always faster.

I know some would prefer one method over the other for reasons other than speed, and I'd be glad to read comments regarding the results, testing method, etc.