Eric Eric - 1 month ago 5
PHP Question

preg_replace PHP not working?

Why doesn't preg_replace return anything in this scenario? I've been trying to figure it out all night.

Here is the text contained within $postContent:

Test this. Here is a quote: [Quote]1[/Quote] Quote is now over.

Here is my code:

echo "Test I'm Here!!!";
$startQuotePos = strpos($postContent,'[Quote]')+7;
$endQuotePos = strpos($postContent,'[/Quote]');
$postStrLength = strlen($postContent);
$quotePostID = substr($postContent,$startQuotePos,($endQuotePos-$postStrLength));
$quotePattern = '[Quote]'.$quotePostID.'[/Quote]';
$newPCAQ = preg_replace($quotePattern,$quotePostID,$postContent);
echo "<br />$startQuotePos<br />$endQuotePos<br />$quotePostID<br />Qpattern:$quotePattern<br />PCAQ: $newPCAQ<br />";


This is my results:

Test I'm Here!!!

35

36

1

Qpattern:[Quote]1[/Quote]

PCAQ:

Answer

For preg_replace(), "[Quote]" matches a single character that is one of the following: q, u, o, t, or e.

If you want that preg_replace() finds the literal "[Quote]", you need to escape it as "\[Quote\]". preg_quote() is the function you should use: preg_quote("[Quote]").

Your code is also wrong because a regular expression is expected to start with a delimiter. In the preg_replace() call I am showing at the end of my answer, that is @, but you could use another character, as long as it doesn't appear in the regular expression, and it is used also at the end of the regular expression. (In my case, @ is followed by a pattern modifier, and pattern modifiers are the only characters allowed after the pattern delimiter.)

If you are going to use preg_replace(), it doesn't make sense that you first find where "[Quote]" is. I would rather use the following code:

$newPCAQ = preg_replace('@\[Quote\](.+?)\[/Quote\]@i', '\1', $postContent);

I will explain the regular expression I am using:

  • The final '@i' is saying to preg_replace() to ignore the difference between lowercase, and uppercase characters; the string could contain "[QuOte]234[/QuOTE]", and that substring would match the regular expression the same.

  • I use a question mark in "(.+?)" to avoid ".+" is too greedy, and matches too much characters. without it, the regular expression could include in a single match a substring like "[Quote]234[/Quote] Other text [Quote]475[/Quote]" while this should be matched as two substrings: "[Quote]234[/Quote]", and "[Quote]475[/Quote]".

  • The '\1' string I am using as replacement string is saying to preg_replace() to use the string matched from the sub-group "(.+?)" as replacement. In other words, the call to preg_replace() is removing "[Quote]", and "[/Quote]" surrounding other text. (It doesn't replace "[/Quote]" that doesn't match with "[Quote]", such as in "[/Quote] Other text [Quote]".)