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Zapping Trailing Commas Using Regular Expressions

(skip to the finished product)

Here's a PHP tip that will hopefully come in handy. Say you have an automatically generated string that dumped a bunch of stuff together and separated them with commas. Meta keywords, post tags, first and last names, whatever. Ideally you could "fix" your generation process and not introduce the trailing comma to begin with, but we all know sometimes life isn't perfect. There are many ways to eliminate that trailing comma.

    // this would be generated somewhere else, it's just assigned here for comprehension
    $generated = "this, that, the other, and something else, ";
    
    ...
    
    $generated = substr($generated, 0, strlen($generated) - 2);
    echo $generated; // this, that, the other, and something else

What if that trailing space isn't there? Sometimes another process may have trimmed it already. Revision number one...

    $generated = "this, that, the other, and something else, ";
    
    if (substr($generated, -1) == ' ') {
        $generated = substr($generated, 0, strlen($generated) - 2);
    } else if (substr($generated, -1) == ',') {
        $generated = substr($generated, 0, strlen($generated) - 1);
    }
    echo $generated; // this, that, the other, and something else

Little messier, but it gets the job done. Is there an easier way?

    $generated = "this, that, the other, and something else, ";
    
    $generated = preg_replace('/, ?$/', '', $generated);
    echo $generated; // this, that, the other, and something else

Back to one line, and now it doesn't matter if there is or isn't a space, or even if the whole trailing ", " isn't found to begin with! It's a short regex, but lets break it down character by character:

  1. / begins a Perl-style regular expression
  2. , is the comma we want to zap
  3. (space) is the space that may or may not already be trimmed
  4. ? means match zero or one of the last character (space)
  5. $ means only match at the end of $generated
  6. / ends a Perl-style regular expression

var tags = [, ];

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