Perl regex matching optional phrase in longer sentence

Question

I\'m trying to match an optional (possibly present) phrase in a sentence:

perl -e \'$_=\"word1 word2 word3\"; print \"1:$1 2:$2 3:$3\\n\" if m/(word1).*(word         


        

Answer 1

In order to solve your issue, you have to observe that the catch-all subexpression in your regex match material that you do not want them to:

 (word1).*(word2)?.*(word3)
        --
         ^--- this subexpression matches _all_ material between `word1` and `word3` in the test string, in particular `word2` if it is present

 (word1).*? (word2)? .*(word3)
        ---+--------+--
         ^       ^   ^-- this subexpression matches _all_ material between `word1` and `word3` in the test string, in particular `word2` if it is present
         |       |
         |       +------ this subexpression is empty, even if `word2` is present:
         |               - the preceding subexpression `.*?` matches minimally (ie. the empty string)
         |               - `(word2)?` cannot match for the preceding blank.
         |               - the following subexpression `.*` matches everything up to `word3`, including `word2`.
         |
         |               -> the pattern matches _as desired_ for test strings
         |                  where `word2` immediately follows `word1` without  
         |
         +-------------- this subexpression will always be empty

What you need is a construction that prevents the catch-all to match strings that contain word2. Luckily, perl's regex syntax sports the negative lookbehind that serves the purpose: for each character in the match of the catch-all subexpression, make sure that it is not preceded by word2.

In perl:

/(word1).*(word2).*(word3)|word1((?

Caveats

1. This might be a performance hog.
2. Note that word2 must be a literal, as the regex engine only supports patterns with match lengths known a priori.

Alternative solution

Given the Caveats you might try to alter the control logic:

$teststring = $_;
if ($teststring =~ m/(word1).*(word2).*(word3)/) {
    print \"1:$1 2:$2 3:$3\n\";
}
else {
    # You know by now that there is no word2 between any word1, word3 occurrences 
    if ($teststring =~ m/(word1).*(word3)/) {
        print \"1:$1 2:- 3:$2\n\";
    }
}