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Re: what is the best way to seperate the digits and strings from variable ?

by polettix (Vicar)
on May 09, 2005 at 10:46 UTC ( #455139=note: print w/ replies, xml ) Need Help??


in reply to what is the best way to seperate the digits and strings from variable ?

Hi swaroop, please post these questions in Seekers of Perl Wisdom instead of Perl Monks Discussion. Also, try to work your examples out a little better: use code tags and quotes around strings :)

Your question is a little vague, but if you want to extract digits at the beginning of a string you can use the following regex:

my $variable = '12345(checkthis)'; $variable =~ /^(\d*)/; my $start_digits = $1;
or, more concisely:
my $variable = '12345(checkthis)'; my ($start_digits) = $variable =~ /^(\d*)/;
Note that in the last example you HAVE to put parenthesys around $start_digits, to force a list/array context evaluation.

Flavio (perl -e 'print(scalar(reverse("\nti.xittelop\@oivalf")))')

Don't fool yourself.


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Re^2: what is the best way to seperate the digits and strings from variable ?
by dbwiz (Curate) on May 09, 2005 at 10:58 UTC

    Please, test your code.

    \d* will match 0 (ZERO) or more digits.

    Thus, it will happily match an empty string at the beginning of a string like "abc123" and return a 0 length string. Try it.

    The right expression to use in this case is \d+.

    Moreover, a capturing regular expression should always be used with a test:

    my $num; my $variable = "abc123"; if ( $variable =~ /(\d+)/) { $num = $1; }
      Please read the OP and the variable names in the test code before slapping hands. You can be right from your implied point of view, but I had my rationale when posting (which implies that I tested my code, of course)

      Here, I'm considering digits as a character class, not as components of a number whose semantic is different from that of a string; this is why I name my variable $start_digits, not $num as you do. As you've surely noted, the OP never talks about numbers, always about digits (anyway, as I noted in my post, the OP was not clear about the usage of this extracted data).

      Thus, when I thought about putting "*" or "+"*, I considered that if there were no digits it was good, and the returned string would be empty.

      I think you can agree with me that, had the OP asked for initial letters, the regex:

      my $variable = "abc123"; my ($letters) = $variable =~ /^([a-zA-Z]*)/;
      would do the job.

      *I swear I thought about that!

      Flavio (perl -e 'print(scalar(reverse("\nti.xittelop\@oivalf")))')

      Don't fool yourself.

        Testing your code is a great concept. Of course, we all have to agree on the specs so we can all agree on what tests are needed. Your code works just fine for a certain subset of possibilities, dbwiz's code works just fine for a different subset of possibilities, both work just fine for the subset of posssibilities as presented by the OP. In the absence of a better spec, we all make assumptions that show the world that we, individually, live in more than they show the world that the OP lives in. (Which is why, if you look back at questions I pose, they're usually quite long-winded - to reduce the "absence of a better spec".)

        As for the initial letters, not that we're straying from the initial thread here ;-), I'd recommend matching with /^([[:alpha:]]*)/ instead. Again, we have to agree on a spec of what "initial letters" means (does it mean English letters, or can it include accented characters, or letters in other non-Roman lettering systems?). If it includes other languages, I like letting perl worry about that stuff for me ;-). Note that it is perfectly reasonable to only accept straight-ascii for some things. We just can't tell from what has been stated so far. (And I've just revealed a bit more about the world I live in.)

        frodo72, Don't fool yourself, as you signature says.

        dbwiz has given you good advice. Assigning $1 without testing is almost a capital sin in Regex parlance. Your initial code would pass a test against the only example provided by the OP, but it would fail in many other cases.

        Cheers

      Please, test your code.

      Testing code in replies is encouraged (if that), but not required. One must simply have the common sense to accept that occasionally untested code will prove itself wrong, and one must be willing to correct it should that occur.

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