Regex to check for beginning character

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

Replies are listed 'Best First'.
Re: Regex to check for beginning character by Limbic~Region (Chancellor) on Mar 05, 2004 at 15:12 UTC
Anonymous Monk, =~ says: matches regex !~ says: does not match regex See perldoc perlop for more info. Either of the following should work: `print "yahoo!\n" if grep /^#/ , @lines; # or for ( @lines ) { if ( /^#/ ) { print "yahoo!\n"; last; } }` [download] The latter stopping on first match. Cheers - LR	[reply] [d/l]
Re: Regex to check for beginning character by perrin (Chancellor) on Mar 05, 2004 at 15:33 UTC
You don't need a regex for an exact match like this. `foreach(@lines) { if ( index( $_, '#' ) == 0 ) { print "yahoo!"; } }` [download] That's much faster than using a regex for this.	[reply] [d/l]
Re^2: Regex to check for beginning character by Nkuvu (Priest) on Mar 05, 2004 at 16:39 UTC
And of course you can simplify this to `foreach(@lines) { print "yahoo!" if ( index( $_, '#' ) == 0 ); }` [download]	[reply] [d/l]
Re: Re^2: Regex to check for beginning character by Anonymous Monk on Mar 05, 2004 at 20:58 UTC
That's not simplification. You're trying to win style points, but that's not how it's done (use parens only when neccessary) :) `for(@lines){ print "yahoo!" if 0 == index $_, '#'; }` [download]	[reply] [d/l]
Re: Regex to check for beginning character by davido (Cardinal) on Mar 05, 2004 at 16:44 UTC
There are a couple of problems. First (in order of appearance), you don't need the /g modifier. You're only looking for the first case in each array element, so /g is useless. Second, !=~ isn't really what you're looking for. If you want it to return true when a match occurs, use =~ If you want it to return false when a match occurs, use !~ Also, if you leave off the =~ operator, $_ is implicit, so you could actually just say `if ( m/^#/ ) {....`. Dave	[reply] [d/l]
Re: Regex to check for beginning character by bart (Canon) on Mar 06, 2004 at 00:23 UTC
I've never seen !=~ before but I assume it's functional. ... except it doesn't do what you want. What you've got here is a combination of two operatores: `!=` and `~`. What you're asking for is to test that $_ is not numerically the same value as the result of `m/^#/g`, bitwise inverted. You can test my claim with the next snippet: `use constant NOT1 => ~ 1; print "~1 is " . NOT1 . "\n"; if(NOT1 !=~ 1) { print "Different (huh?!?)\n"; } else { print "The same (of course!)\n"; }` [download] which results in: ~1 is 4294967294 The same (of course!) For any other number than 1 in the conditional, the test says "different". p.s. Like everybody else seems to have told you already, you're looking for `!~`. Or, you can use the inverted test, using `unless`, instead of `if`	[reply] [d/l] [select]
Re: Regex to check for beginning character by UnderMine (Friar) on Mar 05, 2004 at 17:38 UTC
What about leading whitespace? Assuming you want to return the items starting with a # then :- `@lines = (' #', 'nope', '# yep', 'oops #'); foreach(@lines) { print "yahoo! $_\n" if (/^\s*#/); }` [download] Just a thought UnderMine	[reply] [d/l]
Re: Regex to check for beginning character by ysth (Canon) on Mar 21, 2004 at 05:13 UTC
In the current development version of perl, this will now give a warning: `!=~ should be !~` [download]	[reply] [d/l]
Re: Regex to check for beginning character by slloyd (Hermit) on Mar 05, 2004 at 20:05 UTC
`foreach(@lines){ if($_!~/^\#/s){print "does not start with a pound sign";} else{print "begins with a pound sign\n";} }` [download]	[reply] [d/l]
Re: Regex to check for beginning character by ysth (Canon) on Mar 09, 2004 at 04:09 UTC
Has anyone else ever said !=~ instead of !~ ? Should this give a syntax warning? Newbies especially don't be afraid of speaking up.	[reply]