Search for exact multi-part strings

You must have some delimiters for the regex, and I’m using # to avoid the need to escape the /. You can just as well have / as the delimiters and escape the /.

As for the \1\2\3\4, that’s the replacement for the capturing groups. Because the different patterns have different number of parts, it may be clearer to do multiple separate replacements:

$patterns = array(
  '#(\d{1,3}\-)?(\d{1,3})\s(\w)\s(\d{1,5})/(\d\d)#',
  '#(\w)-(\d{1,4})/(\d\d)#',
  '#(\w{1,2})\s(\w{2,3})\s(\d{1,4})/(\d\d)#',
  '#(\w\w\w)\s(\d+)/(\d+)#',
);
$replacements = array(
  '\1\2\3\4\5',
  '\1\2\3',
  '\1\2\3\4',
  '\1\2\3',
);
$a = preg_replace( $patterns, $replacements, $a );

I also corrected some of your patterns: they had white space and separators inside the capturing groups when the point is to have only the letters and the digits in the capturing groups so that everything else gets filtered out.

But good work on the regexes here!

Thanks and awesome, I think I get what you are saying. To double-check: In the second row, you wrote

'#(\d{1,3}\-)?(\d{1,3})\s(\w)\s(\d{1,5})/(\d\d)#',

Do I not need to de-escape the \-? If so, would this be the final code?

// Custom Fix for Relevanssi plugin to detect court rulings in searches
add_filter( 'relevanssi_remove_punctuation', 'rlv_convert_filenumbers', 9 );
add_filter( 'relevanssi_post_content', 'rlv_convert_filenumbers' );
function rlv_convert_filenumbers( $a ) {
	$patterns = array(
  	'#(\d{1,3}-)?(\d{1,3})\s(\w)\s(\d{1,5})/(\d\d)#',
  	'#(\w)-(\d{1,4})/(\d\d)#',
  	'#(\w{1,2})\s(\w{2,3})\s(\d{1,4})/(\d\d)#',
  	'#(\w\w\w)\s(\d+)/(\d+)#',
	);
	$replacements = array(
  	'\1\2\3\4\5',
  	'\1\2\3',
  	'\1\2\3\4',
  	'\1\2\3',
	);
$a = preg_replace( $patterns, $replacements, $a );
  return $a;
}

Ah, yes, you do need to escape the hyphen. It’s also a good idea to save some individual posts after adding the new filter to see that the file numbers are converted correctly, before reindexing everything.

Great stuff, thanks for your help. Works like a charm!