It's OK, I think I've found the answer to my own question. The # signs bracket the actual regular expression, and the e and the s are modifiers - e is equivalent to PREG_REPLACE_EVAL and  s is equivalent to PCRE_DOTALL. Maybe these are things which are so familiar to the experienced players that they almost go without saying, but ...

I have a problem which seems similar to the one documented here, so I hope somebody reading this can help me too. I have a page of PHP code which works some of the time, but not always, so I'm trying to discover the bugs in it. It includes the line  preg_match_all('#<!-- start content -->(.*?)<!-- end content -->#es', ...