# regular expression

Bengt Richter bokr at oz.net
Sat Mar 26 05:30:21 CET 2005

```On Sat, 26 Mar 2005 02:07:15 GMT, aaron <asteele at berkeley.edu> wrote:

>
>given a string, suppose i wanted to do the following:
>- replace all periods with colons, except for periods with a digit to
>the right and left of it.
>
>for example, given:
>'375 mi. south of U.C.B. is 3.4 degrees warmer'
>
>would be changed to:
>"375 mi: south of U:C:B: is 3.4 degrees warmer'
>
>i was thinking that a regular expression might do the trick. here's what
>i tried:
>!----------------------------------------------------------------------!
>Python 2.4.1c1
>[GCC 3.2.3 20030502 (Red Hat Linux 3.2.3-49)] on linux2
> >>> import re
> >>> pattern = re.compile(r'(?!\d)[.](?!\d)')
> >>> pattern.sub(':', '375 mi. south of U.C.B is 3.4 degrees warmer.')
>'375 mi: south of U:C:B is 3.4 degrees warmer:'
>!----------------------------------------------------------------------!
>
>so this works, but not in the following case:
>!----------------------------------------------------------------------!
> >>> pattern.sub(':', '.3')
>'.3'
>!----------------------------------------------------------------------!
>
>but going the other direction works:
>!----------------------------------------------------------------------!
> >>> pattern.sub(':', '3.')
>'3:'
>!----------------------------------------------------------------------!
>
>any thoughts?
>
Brute force the exceptional case that happens at the start of the line?

>>> import re
>>> pattern = re.compile(r'^[.]|(?!\d)[.](?!\d)')
>>> pattern.sub(':', '375 mi. south of U.C.B is 3.4 degrees warmer.')
'375 mi: south of U:C:B is 3.4 degrees warmer:'
>>> pattern.sub(':', '.3')
':3'
>>> pattern.sub(':', '3.')
'3:'

Seems like an asymmetry in re's handling of (?!\d) after the last char vs before first though.

Regards,
Bengt Richter

```