Stupid Perl Debugger Tricks

I’ve been playing with Python alot lately in my spare time, but I still use mostly Perl at work. One of the handy things about Python is the interactive mode; I like it so much I even cloned it for Perl some time ago. Even without my Perlthon script, you can get a quick approximation in perl using the perl debugger and a command-line script:

perl -de1

(That’s a one, not an el.) The above will invoke perl with the debugger (-d), debugging a very simple script (-e1, which is to say -e '1;'). Once the debugger starts, you can just type perl statements, and can use x <expr> to inspect values.

Whichever way you play with interactive Perl, testing regular expressions can be a pain. It’s not too bad under Python, given Python’s use of match objects:

import re
re.search('regex', 'string').group(0)

The second line runs the regex against the string, and prints the entire match. If your regex doesn’t match at all, you get an error, but that’s fine (and self explanatory). If your regex doesn’t perform as expected, repeated attempts make it easy to triangulate. If you have Python’s readline module installed, you can just hit UpArrow after the test, tweak the regex, lather, rinse, repeat. I wanted the same flexibility with interactive Perl; it turns out to be trivial:

# perl debugger version
x ('string' =~ /regex/, $&)[-1]

# perlthon version
('string' =~ /regex/, $&)[-1];

The regex match operation will return a list of matched groups, which can be handy at times. For testing a complicated regex, I often just want to see the whole match to be sure I’m getting what I expect. The array notation accomplishes this nicely.

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