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I'm trying to create a slug so I would like to strip out every strange character. The only thing the slug should contain is lowercase letters and underscores. Is there a way to check for strange characters and filter it out the string? everything that is not a character or underscore should be deleted

this is what I have:

if(!preg_match_all('/[a-z]/')): $output = preg_replace("/ ... euhm ... /", "", $slug2); else: $output = $slug2; endif;

I should go from this: Create a 3D Ribbon Wrap-Around Effect (Plus a Free PSD!)

to this: create_a_3d_ribbon_wrap_around_effect_plus_a_free_psd

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  • Sometimes it is easier to "remove all characters but this, this, and this"; I think that fits your case. Commented Nov 17, 2010 at 23:02
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    So you also want to translate spaces into underscores? And numbers are also ok? Your example and your description do not match up. Commented Nov 17, 2010 at 23:02
  • There are 1908 lowercase letters, of which your [a-z] comprises merely a hairsbreadth more than 1⅓%. Commented Nov 17, 2010 at 23:48
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    This is a duplicate of possibly many other questions. Here are some: stackoverflow.com/questions/4051889/…, stackoverflow.com/questions/1432463/…, stackoverflow.com/questions/25259/…, stackoverflow.com/questions/3984983/…, etc. Commented Nov 18, 2010 at 6:57
  • @cdhowie of course it does not match, that's why I'm posting the question. Commented Nov 18, 2010 at 8:27

3 Answers 3

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$slug = strtolower($slug); $slug = str_replace(" ", "_", $slug); $slug = preg_replace("/[^a-z0-9_]/", "", $slug);
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4 Comments

I’m afraid you’ve forgotten one thousand eight hundred and eighty-two lowercase letters besides the those quaint 1960sish a-z. ˋunichars -a '\p{Lower}' '[^a-z]' | wc -lˋ == 1882
Based on the OP's example, this looks like it will be used for slugs in a URL. And Unicode characters look ugly in URLs, so I doubt the OP wants to keep them.
Agreed. But interesting fact that there are more than latin, cyrillic and greek. ;)
[^a-z0-9_] could be sensibly reduced to \W.
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No need for the initial match. You can do an unconditional search-and-replace. If there's nothing to replace, no big deal. Here it is as one big chain of function calls:

$slug = trim(preg_replace('/[\W_]+/', '_', strtolower($slug)), '_');

Or split out into separate lines:

$slug = strlower($slug); $slug = preg_replace('/[\W_]+/', '_', $slug); $slug = trim($slug, '_');

Explanation:

  1. Convert uppercase to lowercase with strtolower.
  2. Search for \W and _. A "word" character is a letter, digit, or underscore. A "non-word" character is the opposite of that, i.e. whitespace, punctuation, and control characters. \W matches "non-word" characters.
  3. Replace those bad characters with underscores. If there's more than one in a row they'll all get replaced by a single underscore.
  4. Trim underscores from the beginning and end of the string.

The code's on the complicated side because there are several tricky cases it needs to handle:

  • Bad characters on the ends need to be deleted, not converted to underscores. For example, the !) in your example.
  • We want foo_-_bar to turn into foo_bar, not foo___bar. Underscores should be collapsed, basically.
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$slug = preg_replace("[^a-z_]", "", $slug);
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You forgot almost two thousand lowercase letters.

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