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Regular Expressions: preg_replace only does one per line.

Posted on 2004-11-17
Medium Priority
Last Modified: 2012-06-22
I am writing a little hack for a Wiki that enables highlighting of text using a background color. I use preg_replace to replace text delimited by ++ to insert the appropriate span. It works fine except for one thing, when there are multiple words highlighted in the same line, it only replaces the first and the last ++, leaving the middle ones intact and in effect it highlights the whole line from the first ++ to the last (with ++ as plain text in the middle). Here's the sample code

$text = "test  ++highlight!!++ test test test ++test++ highlight";

echo preg_replace('/\+\+(.*)\+\+/','<span style="background-color: #FFFF00">\\1</span>', $text);

In this case, the first occurence of "highlight" and the last occurence of "test" should be highlighted (ie <span...>highlight</span>). If I insert a \n between the two words somewhere it works as it should.

I am not much of a regular expressions expert so could someone help me fix this one?
Question by:KvdnBerg
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LVL 48

Accepted Solution

hernst42 earned 800 total points
ID: 12603749
echo preg_replace('/\+\+(.*)\+\+/U','<span style="background-color: #FFFF00">\\1</span>', $text);

notify the /U for ungreedy match of preg_replace

Author Comment

ID: 12603818
Perfect, works like a charm but could you please explain in a little more detail why this works?
I usually just copy&paste regex things and adapt them without really understanding them.
LVL 10

Assisted Solution

frugle earned 200 total points
ID: 12604583
preg_replace('/   #start
\+                      # escaped + matches a real +
\+                      # escaped + matches a real +
(.*)                    # everything up to the next match
\+                      # escaped + matches a real +
\+                      # escaped + matches a real +
/                        #stop

'replace with',                                    # \\1 is the contents of the first set of (brackets)


so, in English - look for the first two instances of the plus sign together then read everything until the next set of two plus signs.

Without the /U un-greedy flag, it sees the first and last double-plusses and highlights EVERYTHING in between them.


Author Comment

ID: 12604597
Ok. I did some searching and found out myself. Normally PERL regular expressions are 'greedy', meaning they try to make the longest match possible. The U at the end of hernst42's answer means "ungreedy" so that it makes the shortest match possible. This can also be accomplished by putting a ? after the wildcard (*) like so:

echo preg_replace('/\+\+(.*?)\+\+/','<span style="background-color: #FFFF00">\\1</span>', $text);

See http://www.troubleshooters.com/codecorn/littperl/perlreg.htm#Greedy

LVL 48

Expert Comment

ID: 12604717
See http://de3.php.net/manual/en/reference.pcre.pattern.modifiers.php
the greedyness/ungreedines defines how many chars are used if * are given. The default is to match as many chars as along as the regex matches. If you use ungreedines it tries to match that expression with the minum characters.

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