PHP Preg-Replace more than one underscore

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);

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