steps to debug regular expressions

[Skaperen]

i am trying to make more use of module re. this time i am scanning all the files in (a copy of) /var/log to rename the files after log rotation. but my regular expression is not matching anything. it should match a select few files. is there a common methodology to debug regular expressions?

i'm not showing what i have done so no one is temped to do it for me.

[Larz60+]

there are numerous online testers that you can use, for example: https://regex101.com/
google 'debug regular expressions online' for more

[Gribouillis]

Using the re.VERBOSE flag also helps write expressions on several lines with comments.

[Skaperen]

the re.VERBOSE flag isn't helping any. it lets me write weirder expressions. at least re.DEBUG helped a little.

what seemed to be happening was that it was applying my compiled pattern to the whole string, not just a portion (as seen in things like the egrep command). so i filled out the expression with ^.* and .*$ to make it applicable to the whole string. that made it work.

the re.DEBUG flag was interesting. i made a little command to compile expressions with that flag. it let me see what they get compiled into (presumably in the form of some coding).