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Is there a way to debug a regular expression in Python? And I'm not referring to the process of trying and trying till they work :)
Here is how regexes can be debugged in Perl:
use re 'debug';
my $str = "GET http://some-site.com HTTP/1.1";
if($str =~/get\s+(\S+)/i) {
print "MATCH:$1\n";
}
The code above produces the following output on my computer when ran:
Compiling REx "get\s+(\S+)"
Final program:
1: EXACTF (3)
3: PLUS (5)
4: SPACE (0)
5: OPEN1 (7)
7: PLUS (9)
8: NSPACE (0)
9: CLOSE1 (11)
11: END (0)
stclass EXACTF minlen 5
Matching REx "get\s+(\S+)" against "GET http://some-site.com HTTP/1.1"
Matching stclass EXACTF against "GET http://some-site.com HTTP/1.1" (33 chars)
0 | 1:EXACTF (3)
3 | 3:PLUS(5)
SPACE can match 1 times out of 2147483647...
4 | 5: OPEN1(7)
4 | 7: PLUS(9)
NSPACE can match 20 times out of 2147483647...
24 | 9: CLOSE1(11)
24 | 11: END(0)
Match successful!
MATCH:http://some-site.com
Freeing REx: "get\s+(\S+)"
556
Yes! There's a Python debugger called pdb just for doing that! You can launch a Python program through pdb by using pdb myscript.py or python -m pdb myscript.py . There are a few commands you can then issue, which are documented on the pdb page.
Starting Python Debugger To start debugging within the program just insert import pdb, pdb. set_trace() commands. Run your script normally, and execution will stop where we have introduced a breakpoint. So basically we are hard coding a breakpoint on a line below where we call set_trace().
To validate a field with a Regex pattern, click the Must match pattern check box. Next, add the expression you want to validate against. Then add the message your users will see if the validation fails. You can save time for your users by including formatting instructions or examples in the question description.
To capture all matches to a regex group we need to use the finditer() method. The finditer() method finds all matches and returns an iterator yielding match objects matching the regex pattern. Next, we can iterate each Match object and extract its value.
>>> p = re.compile('.*', re.DEBUG)
max_repeat 0 65535
any None
>>>
regex '|' operator vs separate runs for each sub-expression
145
https://www.debuggex.com is also pretty good. It's an online Python (and a couple more languages) debugger, which has a pretty neat visualization of what does and what doesn't match. A pretty good resource if you need to draft a regexp quickly.
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