preg_match substring只有在有效的情况下才有效

Question

I need to extract a substring from an email subject only if valid. The valid substring is like:

ab-2011-0023ad or ac-2011-0023ad.r1, only two dashes are allowed, number of letters after second dash varies, but only alphanumeric with one dot or without dot.

I wrote this one, but if I put after second dash an illegal character, the regexp returns partial substring not empty

/([a-z0-9]{0,3}[-]\d{4}[-][a-z0-9.]+)\S/i

i.e.: "this is an email with ab-2011-0023:ad document", I expect from regexp to return empty array not [0] => ab-2011-0023.

Answer 1

Your reg exp is not correct. 1. Never use one literal in symbolic class like this [-]. You should escape it -, but nod add to symbolic class. 2. [a-z0-9.]+ this piece matches with string "..............". So, it's not correct. 3. In the end you use as delimiter \S (matches everything except the whitespaces).

My advice is to use smth like this (if your reg exp engine supports Negative Lookbehind)

/[a-z\d]{0,3}\-\d{4}\-(?:[a-z\d]+|(?<!\.)\.)+\s/i

example