Negating a leading whitespace

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

Replies are listed 'Best First'.
Re: Negating a leading whitespace by Limbic~Region (Chancellor) on Jul 05, 2005 at 15:16 UTC
icg, If I understand correctly, you want to do something with a line if it contains 2 characters but exclude it if they are preceded by a whitespace. Negated character classes to the rescue: `if ( $line =~ /[^\s]XY/ ) { ... }` [download] Where XY are the two characters you want. Alternatively, if the whitespace requirement for exclusion is at the beginning of the line you can just do: `while ( <FILE> ) { next if /^\s/; # process lines without leading whitespace here ... }` [download] Cheers - L~R	[reply] [d/l] [select]
Re^2: Negating a leading whitespace by Roy Johnson (Monsignor) on Jul 05, 2005 at 15:23 UTC
Your first example wouldn't match a line that began with XY, because it requires some character to precede it. Negative lookbehind or an alternation would fix: `/(?:^\|\S)XY/ # or /(?<!\s)XY/` [download] (I also prefer the builtin not-whitespace char, rather than negating the class). Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re^2: Negating a leading whitespace by davidrw (Prior) on Jul 05, 2005 at 15:30 UTC
I believe OP wants to check for the 2 chars _anywhere_ in the string, as long as the line doesn't start w/whitespace (so your second method)... I think this is easiest w/just two checks (i was having trouble making a single regex w/look-behinds): `if ( $line =~ /^\S/ && $line =~ /XY/ ){ ... }` [download] Unless you can assume that the 2 chars will never be at the beginning of the line, in which case you can just do: `if ( $line =~ /^\S.*?XY/ ){ ... }` [download]	[reply] [d/l] [select]
Re: Negating a leading whitespace by Anonymous Monk on Jul 05, 2005 at 15:53 UTC
Assume you are looking for "AB", then use: `/^(?!\s).*AB/` [download] or if you can use two regexes: `!/^\s/ && /AB/` [download] the latter is likely to be faster.	[reply] [d/l] [select]
Re: Negating a leading whitespace by dirac (Beadle) on Jul 05, 2005 at 15:44 UTC
#!/usr/bin/perl -w use strict; my $test = "some random text with char X and char Y that I want ... X and Y Y ...\n"; my $occurs = ($test =~ tr/XY//); print "Your letters occurs ",$occurs," times\n";	[reply]