Handling undefined string content

akm2 has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Handling undefined string content by danger (Priest) on Jan 31, 2001 at 22:46 UTC
Use a regex to see if the string contains only spaces (or whitespace, which also includes `[ \t\r\f\n]`): `print "Spaces only\n" if $var =~ /^ +\z/; print "Only whitespace\n" if $var =~ /^\s+$/; print "Whitespace or empty\n" if $var =~ /^\s*$/;` [download] I used \z as the end of string anchor on the first one because you didn't specify whether the string was a line and a \n at the end might not be acceptable.	[reply] [d/l] [select]
Re: Handling undefined string content by arturo (Vicar) on Jan 31, 2001 at 22:40 UTC
Use a regular expression: `if ($var =~ /^\s$/) { # do something }` [download] The regex means "if $var consists of nothing but 0 or more spaces between the beginning and the end ..." (for more on that, type `perldoc perlre` on your system) Beware : defined has a meaning in Perl. Philosophy can be made out of anything. Or less* -- Jerry A. Fodor	[reply] [d/l] [select]
Re: Handling undefined string content by Trinary (Pilgrim) on Jan 31, 2001 at 22:42 UTC
`if ($var =~ /^\s+$/) { #do something. }` [download] Explanation: the =~ makes a regular expression bind against a var, instead of $_ / / delimit the regexp. ^ denotes the beginning of a string, $ the end. \s+ means one or more whitespace characters, which is space, \n, \t, \v, or \r. So the whole thing means: do something if, in this var, there's nothing but whitespance between the beginning and end of the contents. Check out perlre. Regular expressions are good. Enjoy Trinary	[reply] [d/l]
Re: Handling undefined string content by jeroenes (Priest) on Jan 31, 2001 at 23:00 UTC
`if (length(substr($var, ' ')) eq length( $var)) {}` might be considered as well. Jeroen "We are not alone"(FZ) Update Read lemming's reply. Forget the substr approach. Luckily, I thought of another approach on the way home: `if( $var eq ' ' x length($var) ) {...}` [download] Should do the trick without regex or functions. Yet another would be to use tr: `if( $var eq map tr/A..Za..z0..9/ /, $var ) {..}` [download] or make that `if( $var eq map tr/\000..\277/ /, $var ) {..}` [download] to be safe. Update2: lemming's update hits it once more. I confused the foreach behavior with map's. Moreover, I should have evaluated in array context. Well, you get this: `if($var eq join '', map{ tr/\000..\177/ /c; $_; } split '', $var ) { +...}` [download] This is tested and it works. Note the use of c to complement. I wouldn't use this myself, though.	[reply] [d/l] [select]
Re: Re: Handling undefined string content by lemming (Priest) on Feb 01, 2001 at 00:24 UTC
Drat. jeroen took off just as I spotted his possible typo. Hopefully, he'll correct it. `if (length(substr($var, index($var, ' '), rindex($var, ' ')+1)) == len +gth($var) { Dosomething(); }` [download] Not that I'd advocate that approach. The rindex+1, could have length instead, but I still wouldn't use it over the regex answers On to what's wrong with jeroen's statement: sorry `substr($var,' ')` will always return the full string since `' '` will be interpreted as 0. use warnings will point out not a numeric argument. Also since length returns numeric values, == instead of eq should be used. Not that this would happen, but "010" eq "10" is false while "010" == "10" is true. Update: Ah good. He saw it, the length x space works, the tr solution does not. tr returns the number of matches to a scalar. So if you had 10 spaces in your string, you would get the number 10. And of course these would only we applicable to space answers, not on any whitespace.	[reply] [d/l] [select]
Re: Handling undefined string content by Fastolfe (Vicar) on Feb 01, 2001 at 03:24 UTC
If you're going to go the regex approach, I might test for the presence of a non-whitespace character instead of looking to see that the string is composed entirely of whitespace. Generally this will be faster: `if ($string !~ /\S/) { # $string is empty or composed entirely of spaces }` [download] You might use length and/or the "trueness" of the string if you want to test if it has content at all.	[reply] [d/l]


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