Re^5: Selecting HL7 Transactions

[kcott]

Ignoring the issue with newlines and the "s" modifier that I alluded to earlier, the heart of the matter is that "." matches any character while "[^|]" matches any character except the pipe character.

Taken in isolation, /(.*?\|)/ and /([^|]*\|)/ may well produce the same result:

$ perl -Mstrict -Mwarnings -E '
    my $x = q{A|||||Z};
    my $dot_re = qr{(.*?\|)};
    my $cc_re = qr{([^|]*\|)};
    $x =~ $dot_re;
    say $1;
    $x =~ $cc_re;
    say $1;
'
A|
A|

The reasons they do this, however, are different. "A" is the least number [non-greedy] of zero or more of any characters (".*?") that match before a literal pipe character ("\|"). It just so happens that "A" is also the greatest number [greedy] of zero or more non-pipe characters ("[^|]*") that match before a literal pipe character ("\|"). So, in both cases "A|" is captured.

Now consider the following where the capture groups are no longer in isolation:

$ perl -Mstrict -Mwarnings -E '
    my $x = q{A|||||Z};
    my $dot_re = qr{(.*?\|)Z};
    my $cc_re = qr{([^|]*\|)Z};
    $x =~ $dot_re;
    say $1;
    $x =~ $cc_re;
    say $1;
'
A|||||
|

Here, "A||||" is the least number [non-greedy] of zero or more of any characters (".*?") that match before a literal pipe character ("\|") that is immediately followed by a literal Z character: "A||||" plus "|" are captured. However, "" (i.e. nothing) is the greatest number [greedy] of zero or more non-pipe characters ("[^|]*") that match before a literal pipe character ("\|") that is immediately followed by a literal Z character: "" plus "|" are captured.

I recommend you take a look at Regexp::Debugger which provides a visualisation of Perl's regular expression engine in action — I think you'll find it most enlightening.

I'd also recommend you look at the Perl documentation (available online at http://perldoc.perl.org/perl.html) before reaching for an internet search engine. Here's a list of Perl regular expression documentation that you'll find linked from that page:

• perlrequick — Perl regular expressions quick start
• perlretut — Perl regular expressions tutorial
• perlfaq6 — Perl frequently asked questions: Regexes
• perlre — Perl regular expressions, the rest of the story
• perlrebackslash — Perl regular expression backslash sequences
• perlrecharclass — Perl regular expression character classes
• perlreref — Perl regular expressions quick reference

That's the order the links appear on that page: look at them in whatever order you want. To be honest, I was a little surprised there was so many; had I realised in advanced, I might not have chosen to start enumerating them all here.

[BillDowns]

Frankly, I find the Perl documentation quite badly written years ago and I gave up on it.

Back to the non-greedy matching, I understand what you are saying. It's just that's not the normal and usual understanding of the word. To my mind, and I would bet most of the world, since the expression was (*.?\|) that would mean the least number of characters before the next - stress that next - \|.

That's what "non-greedy" should mean, IMO. Perl's implementation still greedy.

[Anonymous Monk]

Frankly, I find the Perl documentation quite badly written years ago and I gave up on it.

Um, that doesn't sound reasonable :/