Question

I am trying to figure out how to capture one statement if the other one doesn’t exist using preg_match.

Sample Text:

<!-- InstanceBeginEditable name="doctitle" -->

<title>BU Libraries | Research Guides | Citing Your Sources</title>

<!-- InstanceEndEditable -->

<div id="standardpgt"><h1><!-- InstanceBeginEditable name="pagetitle" --><strong>Citing Your Sources</strong><!-- InstanceEndEditable --></h1></div>

Because pagetitle exists I want to pull it instead of the doctitle tag. Of course there is tons of other characters in between them, but I wanted to show you a small sample.

If pagetitle didn’t exist I would want to grab the contents of doctitle.

The twist is that I’m not using the php code directly, I’m passing in a regex statement through a config file, then a script is taking it and pulling out the 1st group from the statement.

This is what I came up with:

((?!.*?<!--\s*?InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->.*?<!--\s*?InstanceEndEditable\s*?-->)<!--\s*?InstanceBeginEditable\s*?name=\x22doctitle\x22\s*?-->\s*?<title>(.*?)<\/title>\s*?<!--\s*?InstanceEndEditable\s*?-->|<!-- InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->(.*?)<!--\s*?InstanceEndEditable\s*?-->)

What the issue is for some reason php always reads the first empty group as group 1 if it didn’t work.

For example in the sample text above it would return

0 -> <!-- InstanceBeginEditable name="pagetitle" --><strong>Citing Your Sources</strong><!-- InstanceEndEditable -->
1 -> 
2 -> <strong>Citing Your Sources</strong>

I can’t for the life of figure out how to make this work. I also wrote this regex:

(?(?=.*?<!--\s*?InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->.*?<!--\s*?InstanceEndEditable\s*?-->).*?<!-- InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->(.*?)<!--\s*?InstanceEndEditable\s*?-->|.*?<!--\s*?InstanceBeginEditable\s*?name=\x22doctitle\x22\s*?-->\s*?<title>(.*?)<\/title>\s*?<!--\s*?InstanceEndEditable\s*?-->)

But that didn’t work either. Thank you very much for the help.

Chris

Answer 1

user178551 is absolutely correct in recommending the use of a branch reset construct. There is fundamentally nothing wrong with your original regex (other than the fact that it is more than 300 characters long and is ALL ON ONE LINE! – and that it is unable to put one of two alternatives in a single capture group). A non-trivial (to put it mildly) regex like this needs to be written in free-spacing mode with indentation so you can actually read it. Here is your original regex with some reasonable whitespace added:

$re_OP1 = '%
    (                                             # $1:
      (?!
        .*?<!--\s*?InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->
        .*?<!--\s*?InstanceEndEditable\s*?-->
      )
           <!--\s*?InstanceBeginEditable\s*?name=\x22doctitle\x22\s*?-->\s*?
           <title>(.*?)<\/title>\s*?              # $2: 
           <!--\s*?InstanceEndEditable\s*?-->
    |      <!-- InstanceBeginEditable\s*?name=\x22pagetitle\x22\s*?-->
           (.*?)                                  # $3;
           <!--\s*?InstanceEndEditable\s*?-->
    )
    %six';

Looking at this regex now, you can see where you have hard coded one space on the line with the OR operator (i.e. |<!-- InstanceBegin...). This will cause the regex to fail to match with the 'x' modifier is applied. So replacing this space with a \s* and running it on your test data, here are the result I get (php-5.2.14):

Array
(
    [0] => <!-- InstanceBeginEditable name="pagetitle" --><strong>Citing Your Sources</strong><!-- InstanceEndEditable -->
    [1] => <!-- InstanceBeginEditable name="pagetitle" --><strong>Citing Your Sources</strong><!-- InstanceEndEditable -->
    [2] =>
    [3] => <strong>Citing Your Sources</strong>
)

These results are similar to the ones you posted (but for some reason your results show only 2 capture groups???) All we need to do now is to apply user178551’s branch reset suggestion, and the regex solution becomes:

$re_jmr = '%
    (?|  # Branch reset construct. (restart counting for each alternative)
      (?!
        .*?<!--\s*InstanceBeginEditable\s*name="pagetitle"\s*-->
        .*?<!--\s*InstanceEndEditable\s*-->
      )
           <!--\s*InstanceBeginEditable\s*name="doctitle"\s*-->\s*
           <title>(.*?)<\/title>\s*              # $1: Group 1A
           <!--\s*InstanceEndEditable\s*-->
    |      <!--\s*InstanceBeginEditable\s*name="pagetitle"\s*-->
           (.*?)                                  # $1: Group 1B
           <!--\s*InstanceEndEditable\s*-->
    )
    %six';

I’ve gone ahead and changed all the lazy \s*? to greedy (because greedy is what you want here). I also changed all the \x22 to just " – shorter and more readable IMHO. And here are the results from running with this new, branch reset regex:

Array
(
    [0] => <!-- InstanceBeginEditable name="pagetitle" --><strong>Citing Your Sources</strong><!-- InstanceEndEditable -->
    [1] => <strong>Citing Your Sources</strong>
)

Which is, (if I’m not mistaken), exactly what you are looking for. (You did not provide a test case for the other alternative so that has not yet been tested.) Other than that, your original regex was pretty close.