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Re: regex at word boundary

by mikeraz (Friar)
on Dec 07, 2005 at 22:06 UTC ( #515060=note: print w/replies, xml ) Need Help??

in reply to regex at word boundary

I reviewed the link that swkronenfeld provided. The code samples, or at least as many as I waded through, did not find palindromes. The found mirror strings. I prefer pi is a palindrome. aaabbcbbaaa is not a palindrome.

After writing to steele1381 and QM about intent and all that I decided I'd better step up and show the code.

I believe this word end searching should be faster than a pure regex version but have not tested it yet.

Here's the trivial program I wrote to take a text file and locate all the palindromes. Note: it's trivial and should be easy to break. In particular the punct filter is incomplete, HTML will break it . . .

#!/usr/bin/perl -w use strict; # find palindromes in text file my ($le, @lines, @F, $test, $pal, %pals, $start_char, $i, $word); # cross line boundries but not paragraph boundries $le = $/; $/ = "\n\n"; @lines = <>; $/ = $le; foreach (@lines) { # strip punct - no posix on my system s/[)(\?".,\/]//g; s/-/ /g; chomp; (@F) = split; while (int @F) { # select array slices where last letter of last word in # slice equals first letter of first word $start_char = lc substr $F[0], 0, 1; foreach $i (1 .. $#F) { if( (lc substr $F[$i], -1) eq $start_char) { # test for palindrome $test = lc join "", @F[0..$i]; if($test eq reverse $test) { $pal = join " ", @F[0..$i]; $pals{$pal}++; } } } # grab single word palindromes $word = shift @F; if(length $word > 2 && lc $word eq lc reverse $word) { $pals{$word}++; } } } foreach $pal (sort keys %pals) { print "$pals{$pal}\t$pal\n"; }
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