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Re^5: Selecting HL7 Transactions

by kcott (Abbot)
on May 02, 2013 at 02:41 UTC ( #1031683=note: print w/ replies, xml ) Need Help??


in reply to Re^4: Selecting HL7 Transactions
in thread Selecting HL7 Transactions

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.

-- Ken


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Re^6: Selecting HL7 Transactions
by BillDowns (Novice) on May 02, 2013 at 03:07 UTC

    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.

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

      Um, that doesn't sound reasonable :/

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