[Tutor] Letters & Digits [regular expressions]

[Danny Yoo]

On Fri, 16 Nov 2001, Lee-Shanok, Bruce wrote:

> I know there are constants in Python that define all the digits,
> letters, printable characters, etc. What I'm wondering is if there's a
> clean and easy way to test each character in a string to see if they
> match (or do not match) characters from those lists. So far the best
> I've been able to come up with are sequences of for loops, but I'm
> imagining there must be a better way.

Hmmm... I know I should try to resist the urge to mention regular
expressions, but it's too hard!  *grin*


Regular expressions may help you in this case --- they're used to find
specific patterns in text.  Here's one example that may help you:

###
>>> only_digits = re.compile(r"""  ^      # at the very start...           
...                                \d+    # we only want digits!
...                                $      # Even up to the end.      
...                           """, re.VERBOSE)
>>> only_digits.search("42")
<SRE_Match object at 0x8131c70>
>>> only_digits.search("hello")
>>> only_digits.search("he42llo")
>>> only_digits.search("1234567890")
<SRE_Match object at 0x81311e0>
###

The "only_digits" regular expression knows how to recognize a digit
string.  You should be able to modify this example to make it a little
smarter.


You can find out more about regular expressions in the regex HOWTO on
Python.org:

    http://py-howto.sourceforge.net/regex/regex.html


Also, there's a great regular expression tutorial from the Perl folks at:

    http://www.perl.com/CPAN-local/doc/FMTEYEWTK/regexps.html

It's a fairly fun piece to read, and the concepts in the tutorial there
transfers nicely to Python.


Regular expressions are very powerful, and that's the problem: sometimes,
they're just so seductive that they obscure potentially better solutions.


Hope this helps!