Regex Quantifiers

FFSparky has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Regex Quantifiers by kennethk (Abbot) on Jul 15, 2009 at 19:39 UTC
Your problem is your initial greedy .* - that grabs as much as is can, leaving the minimum match for the next term. I don't follow why you would want .* (particularly since you don't use either a ^ or $ anchor). You will get your expected results with either `($Regex1) = ($String =~ /.?(\d{6,8})./);` or `($Regex1) = ($String =~ /(\d{6,8})/);` See perlre or perlretut for details, in particular Matching repetitions. As a side note, please wrap code in <code> tags - see Markup in the Monastery for details.	[reply] [d/l] [select]
Re: Regex Quantifiers by toolic (Bishop) on Jul 15, 2009 at 19:40 UTC
($Regex1) = ($String =~ /.(\d{6,8})./); I suspect your first `.` is being too greedy* and eating up the 1st 2 digits from your `\d{6,8}` range. This gives your 8 digits: `($Regex1) = ($String =~ /(\d{6,8})/);` [download]	[reply] [d/l] [select]
Re: Regex Quantifiers by NetWallah (Canon) on Jul 15, 2009 at 19:44 UTC
The initial "." in $Regex1 is greedy, grabbing as much as it can. You can make it less greedy by adding "?" - this works: `my ($Regex1) = ($String =~ /.?(\d{6,8}).*/);` [download] Update: Multiple collisions on this reply! Potentia vobiscum ! (Si hoc legere scis nimium eruditionis habes)	[reply] [d/l]
Re: Regex Quantifiers by AnomalousMonk (Archbishop) on Jul 15, 2009 at 19:45 UTC
The first `.` in the regex `/.(\d{6,8})./` immediately consumes all characters in the string. The regex then starts backtracking to try to find a match for 6 to 8 digits. It finds such a match in the 6 digits of `'345678'`. The second `.` in the regex then tries to match zero or more of anything and, as one would expect, finds a match. The overall successful match then terminates.	[reply] [d/l] [select]
Re: Regex Quantifiers by Anonymous Monk on Jul 15, 2009 at 19:52 UTC
If you're wondering what regex mean, try YAPE::Regex::Explain use YAPE::Regex::Explain; print YAPE::Regex::Explain->new( qr/.(\d{6,8})./ )->explain,"\n"; __END__ The regular expression: (?-imsx:.(\d{6,8}).) matches as follows: NODE EXPLANATION ---------------------------------------------------------------------- (?-imsx: group, but do not capture (case-sensitive) (with ^ and $ matching normally) (with . not matching \n) (matching whitespace and # normally): ---------------------------------------------------------------------- .* any character except \n (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- \d{6,8} digits (0-9) (between 6 and 8 times (matching the most amount possible)) ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- .* any character except \n (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download] To see what is happening, try `use re 'debug';`	[reply] [d/l]
Re: Regex Quantifiers by FFSparky (Acolyte) on Jul 15, 2009 at 20:23 UTC
Everyone, Thank You for your Wisdom! I was being greedy trying to pattern the entire string as opposed to just what I wanted. And also for the additional reference links and debugging methods! Very much appreciated!	[reply]
Re: Regex Quantifiers by biohisham (Priest) on Jul 15, 2009 at 20:36 UTC
do you know that when you are using the concatenation operators you are actually embedding into a string what is being concatenated to it and since Perl allows you to do string interpolation on the dereferenced variables you could well write this `print "\n\n\$String = ".$String."\n\n";` [download] as this: `print "\n\n\$String = $String\n\n";` [download] and the same thing goes for `print "\$Regex1 = ".$Regex1."\n";` [download] and `print "\nJust to check:\n\$1 = ".$1."\n"` [download] so try to be more organized in writing your code... I see an HTML line break in the body of your code !!!! Excellence is an Endeavor of Persistence. Chance Favors a Prepared Mind	[reply] [d/l] [select]