regex hell

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

I'm really quite confused here. Hopefully, someone can give some idea of what I'm doing wrong. Here is the snippet of code I'm using:

if ($plat =~ /[^a-z]/) {
       print "Nothing here.\n";    
       $change = $false;
}

if ($change) {
      print "Platform: $plat*\n";
}
[download]

Output of my program:
Platform: *

Doesn't that really suggest that there was no lowercase letter in the variable $plat????
sometimes it works right and I'll get the intended result:
Nothing here.

Any ideas are appreciated.

Comment on regex hell Download Code

Replies are listed 'Best First'.
Re: regex hell by davido (Cardinal) on Mar 01, 2006 at 01:48 UTC
You're mistaking character class negation for "doesn't match". `[^a-z]` means "Match a single character that is not a through z." But it doesn't mean "target may not contain a-z." If that's your intention, you might accomplish it like this: `if( $plat !~ m/[a-z]/ ) { ....` Or... `if( $plat =~ m/^[^a-z]*$/ ) { ...` The former simply says "If target doesn't match a-z." The second says, "If target matches a string that contains any amount of anything except for a-z." Dave	[reply] [d/l] [select]
Re^2: regex hell by diotalevi (Canon) on Mar 01, 2006 at 01:58 UTC
Your second example is horrible. Say `not /[a-z]/`, never `/^[^a-z]*$/`. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply] [d/l] [select]
Re: regex hell by GrandFather (Saint) on Mar 01, 2006 at 01:02 UTC
I'm pretty confused too. I have very little idea what you are trying to achieve. However if you modify the code below and test strings to demonstrate your problem, maybe we can help you further? Note that the regex you specify matches any string in which there is at least one character which is not a lower case alpha character. `use strict; use warnings; my $change = 1; while (my $plat = <DATA>) { chomp $plat; print "$plat: "; if ($plat =~ /[^a-z]/) { print "Nothing here."; $change = 0; } if ($change) { print "Platform: $plat*"; } print "\n"; } __DATA__ OS/x os/x OS/X win WIN Win` [download] Prints: `OS/x: Nothing here. os/x: Nothing here. OS/X: Nothing here. win: WIN: Nothing here. Win: Nothing here.` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re: regex hell by ysth (Canon) on Mar 01, 2006 at 09:45 UTC
Show more of your code? Where is $change ever set true? What is $false? Also, try: `$plat = "asdfb\tssf"; use Data::Dumper; $Data::Dumper::Useqq=1; $Data::Dumper::Terse=1; print "Platform: ", Dumper($plat), "\n";` [download] to see what's in $plat in a more definitive way.	[reply] [d/l]