Question

I’ve got this pattern which I use with preg_match and I can’t figure where I go wrong.

$pattern = "{(\[fn:)([0-9]*)(\])((?:\\\[|[^\[])*)}";

It has to match each [fn:i]text_multiline untill another one starts so when it meets [ and i want to escapte the [ by \[ from the following example:

[fn:1] This is the text for \[this needs to be escaped] footnote 1.

Note that it could be multiple paragraphs.

[fn:2] This is the text for footnote 2.

This is the matches that I get at the moment:

array(5) {
  [0]=>
  string(6) "[fn:1]"
  [1]=>
  string(4) "[fn:"
  [2]=>
  string(1) "1"
  [3]=>
  string(1) "]"
  [4]=>
  string(0) ""
}

Answer 1

The technique to use when you want to exclude and escaped [ is called “negative look behind”.

For example,

(?<!\\)\[

This regex will match [ but ignore if its proceeded by a \

Let me know if you need any more help

Edit #1

This is the negative lookbehind applied to your specific scenario

\[fn:\d+](?:\\\[|[^[])*

and in php this becomes

if (preg_match('/\[fn:\d+\](?:\\\\\[|[^[])*/', $subject)) {
    # Successful match
} else {
    # Match attempt failed
}

Some notes

• I use alternation and let it first match the escaped [ so it doesn’t get excluded by what matches after the alternation (common trick that relies on the order in alternation)
• I started from scratch so if you need to capture some groups then add parenthesis where needed
• The construct (?:) is used to group but not to capture. This is (maybe) more efficient and as said I left out the capturing

The regex is tested against your example. Let me know if you can get it working now

Good luck, Buckley