Now let me explain zero-width lookaheads so you know what that code is about. When you do a match against $string, Perl keeps track of the offset from the start of the string, which you can get (or set, actually) with pos($string). It makes more sense when you are doing multiple matches against the same string. Let's say I want to collect all the peppers in the following:
This will put 4 peppers in the array @peppers. As you probably know, the /g modifer tells the match to remember the position after the last match, and the next time through the loop, start looking for another match from that point. So...$s = "I'm a pepper, he's a pepper, you're a pepper, she's a pepper.. +."; while( $s =~ /(pepper)/g ) { push @peppers, $1; }
...the first time through the loop, pos($s) is 0. After matching the first time, the offset is 12, at position 1. After the next match, the offset is at position 2, and so on until there are no more matches.I'm a pepper, he's a pepper, you're a pepper, she's a pepper... ^ ^ ^ ^ ^ 0 1 2 3 4
A zero-width lookahead means a) do the match and b) if successful, put the offset back where it was before you started. So, in this example...
...the regex starts searching from the start of the line by default and matches pepper. But it doesn't change the offset to position 1. Instead, it leaves it at position 0, where it searches forward to find 'like'. $s =~ /pepper.+like/ would have failed because after matching pepper, the offset would be at position 1, and searching forward won't find 'like'. The code is the equivalent of $s =~ /pepper.+like|like.+pepper/. It's more useful when parsing complex phrases, like language, where a verb, for example, can be followed by more than one type of word or phrase.$s = "Wouldn't you like to be a pepper too?"; # ^ ^ # 0 1 $s =~ /(?=pepper).+like/;
But getting back to your post:
A negative lookahead is like the positive lookahead, above, but succeeds when the search term is not found. In the middle of the regex above, I want the match to succeed if there are two quotes, but they do not contain 'Furry Marmot'. It won't work if I try to match "(?!.+Furry Marmot)" because that says a) find a double-quote, b) don't find 'Furry Marmot' and then leave the offset just after the quote, and c) find the closing quote. This can only match "".print "Spam!!!\n" if $text =~ /^To: \s* " (?!.+Furry Marmot) [^"]* " <marmot\@furrytorium\.com> /mx;
Instead, once we have determined that Furry Marmot is not after the first quote, match zero or more of anything that isn't another quote, up to the closing quote. Now we can check what's in the email address.
This is just a simplistic example, and probably would be overkill if you tried to accomodate multiple addressees, a CC: or BCC: line, etc., but I hope it helps you learn regexes. They are one of my favorite parts of Perl. :-) There's a very good description of backtracking in perldoc perlretut. That and perldoc perlre will shed a lot of light on this.--marmot
In reply to Re^5: Looking for ideas on how to optimize this specialized grep
by furry_marmot
in thread Looking for ideas on how to optimize this specialized grep
by afresh1
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