Ideas to improve this regexp?

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

I'm looking for a regexp which matches if the string is ~~non-empty AND~~ either empty or if the first non-space character in the string is not a digit. The string is guaranteed to contain no \n.

Of course I can do it with

/^\s*[^\s\d]|^$/
[download]

but I wonder if there are simpler/more elegant solutions.

updated correction about handling of empty string

--
Ronald Fischer <ynnor@mm.st>

Comment on Ideas to improve this regexp? Download Code

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Re: Ideas to improve this regexp? by moritz (Cardinal) on Nov 03, 2009 at 13:43 UTC
Your regex does match the empty string, which it should not from your descriptions. So remove the `\|^$` part. You can also write your regex as `/\A(?>\s)\D/`, where `(?>\s)` means grab all whitespaces and don't give them up for backtracking. I don't know if you consider that more simple or elegant, though. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
Re^2: Ideas to improve this regexp? by rovf (Priest) on Nov 03, 2009 at 14:17 UTC
Your regex does match the empty string, which it should not from your descriptions. My mistake in wording the question. I've updated the post. I like your alternative suggestion too, but as you already suspected, it is not much simpler anyway. -- Ronald Fischer <ynnor@mm.st>	[reply] [d/l]
Re: Ideas to improve this regexp? by johngg (Canon) on Nov 03, 2009 at 14:37 UTC
This seems to work if you don't let the engine give any spaces back by switching off backtracking. `$ perl -le ' > print > qq{->$_<-: }, > m{(?x) ^ (?: \z \| (?> \s* ) \D ) } > ? q{matches} > : q{no match} > for q{}, q{12ab}, q{cat}, q{ 99balloons}, q{ dog};' -><-: matches ->12ab<-: no match ->cat<-: matches -> 99balloons<-: no match -> dog<-: matches $` [download] I hope this is of interest. Cheers, JohnGG	[reply] [d/l]
Re: Ideas to improve this regexp? by gmargo (Hermit) on Nov 03, 2009 at 13:54 UTC
~~`/^\s*\D/`~~ ~~[download]~~ Update: Thank you for the correction, moritz.	[reply] [d/l]
Re^2: Ideas to improve this regexp? by moritz (Cardinal) on Nov 03, 2009 at 13:59 UTC
That doesn't work as expected. If the string starts with a whitespace followed by a digit, \s* matches the empty string and \D the whitespace, so you'll get false positives.	[reply]