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### Re^3: Selecting HL7 Transactions

by kcott (Chancellor)
 on May 02, 2013 at 00:20 UTC ( #1031672=note: print w/replies, xml ) Need Help??

in reply to Re^2: Selecting HL7 Transactions

"The parentheses are used for the repeat factor. All my research on the internet indicates a multi-character pattern that needs to be repeated multiple times should be enclosed in parentheses. Is this not correct?"

Here's a test showing clustering and capturing. Both match as expected. Capturing also sets \$1.

```\$ perl -Mstrict -Mwarnings -E '
my \$re1 = qr{PV1\|1\|O\|(?:[^|]*\|){3}\|};
my \$re2 = qr{PV1\|1\|O\|([^|]*\|){3}\|};
my \$x = q{PV1|1|O|F3|F4|F5|F6|F7};
my \$y = q{PV1|1|O|F3|F4|F5||F7};
say "------- Clustering -------";
say "Match in \\$x" if \$x =~ /\$re1/;
say \$1 if \$1;
say "Match in \\$y" if \$y =~ /\$re1/;
say \$1 if \$1;
say "------- Capturing -------";
say "Match in \\$x" if \$x =~ /\$re2/;
say \$1 if \$1;
say "Match in \\$y" if \$y =~ /\$re2/;
say \$1 if \$1;
'
------- Clustering -------
Match in \$y
------- Capturing -------
Match in \$y
F5|

-- Ken

Replies are listed 'Best First'.
Re^4: Selecting HL7 Transactions
by BillDowns (Novice) on May 02, 2013 at 01:02 UTC

On a different note, since you seem quite knowledgeable about Perl, and again referring to google searches, non-greedy matching is usually defined in a manner that  (.*?\|) and  ([^|]*\|) should be equivalent. And I think so, too. Why are they not?

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

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.

Re^4: Selecting HL7 Transactions
by Anonymous Monk on May 02, 2013 at 00:46 UTC
Thanks for that info - I did not know about clustering. It does not show up in the first half-dozen google searches on regular expressions. In this case, it doesn't matter - the utility script doesn't care about any capturing. It is simply qualifying.

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