what the pattern searches for

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

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Re: what the pattern searches for by ysth (Canon) on Dec 17, 2004 at 10:17 UTC
Sure. To get an annotated description: `use YAPE::Regex::Explain; my $pattern='(\n\s[0-9]{1,3}\s\n\s<page>\s\n\|\n\s-[0-9]{1,3}-\s\ +n\s<page>\s\n\|\n\s[A-Za-z]-[0-9]{1,3}\s\n\s<page>\s\n\|\n\spage +\s[0-9]{1,3}\sof\s[0-9]{1,3}\s\n)'; print YAPE::Regex::Explain->new($pattern)->explain;` [download] which produces: The regular expression: (?-imsx:(\n\s[0-9]{1,3}\s\n\s<page>\s\n\|\n\s-[0-9]{1,3}-\s\n\s< +page>\s\n\|\n\s[A-Za-z]-[0-9]{1,3}\s\n\s<page>\s\n\|\n\spage\s[0 +-9]{1,3}\sof\s[0-9]{1,3}\s\n)) 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): ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- [0-9]{1,3} any character of: '0' to '9' (between 1 and 3 times (matching the most amount possible)) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- <page> '<page>' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \| OR ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- - '-' ---------------------------------------------------------------------- [0-9]{1,3} any character of: '0' to '9' (between 1 and 3 times (matching the most amount possible)) ---------------------------------------------------------------------- - '-' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- <page> '<page>' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \| OR ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- [A-Za-z] any character of: 'A' to 'Z', 'a' to 'z' ---------------------------------------------------------------------- - '-' ---------------------------------------------------------------------- [0-9]{1,3} any character of: '0' to '9' (between 1 and 3 times (matching the most amount possible)) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- <page> '<page>' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \| OR ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- page 'page' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- [0-9]{1,3} any character of: '0' to '9' (between 1 and 3 times (matching the most amount possible)) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- of 'of' ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- [0-9]{1,3} any character of: '0' to '9' (between 1 and 3 times (matching the most amount possible)) ---------------------------------------------------------------------- \s* whitespace (\n, \r, \t, \f, and " ") (0 or more times (matching the most amount possible)) ---------------------------------------------------------------------- \n '\n' (newline) ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download] Now, if you would tell us something about the context in which this is used (other than just that it is used on html), maybe a higher-level description would be possible.	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Re: what the pattern searches for by BrowserUk (Patriarch) on Dec 17, 2004 at 10:24 UTC
It appears to be looking for and capturing lines containing page numbers in one of 4 forms. Roughly: `' nnn <page> ' ' - nnn - <page> ' ' A-nnn <page> ' ' page nnn of mmm '` [download] This becomes fairly clear if you break up and expand the regex a little using /x. `my $pattern = qr[ ( \n\s* [0-9]{1,3} \s\n\s <page> \s\n \| \n\s - [0-9]{1,3} - \s\n\s <page> \s\n \| \n\s [A-Za-z]-[0-9]{1,3} \s\n\s <page> \s\n \| \n\s page \s* [0-9]{1,3} \s* of \s* [0-9]{1,3} \s\n ) ]x;` [download] There are many ways the regex could be improved, but that isn't what you asked :) Examine what is said, not who speaks. The end of an era!* "But you should never overestimate the ingenuity of the sceptics to come up with a counter-argument." -Myles Allen "Think for yourself!" - Abigail "Time is a poor substitute for thought"--theorbtwo "Efficiency is intelligent laziness." -David Dunham "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]