sre_parse

Internally, the module re uses 2 undocumented libraries:

• sre_parse to parse regular expressions into an abstract syntax tree.
• sre_compile to compile parsed expression.

The first one can be used to see how a regexp was parsed by Python. There are many better tools and services (like regex101.com) to debug regular expressions but this one is already in the stdlib.

>>> import sre_parse
>>> sre_parse.parse(r'([Pp]ython)\s?etc').dump()
SUBPATTERN 1 0 0
  IN
    LITERAL 80
    LITERAL 112
  LITERAL 121
  LITERAL 116
  LITERAL 104
  LITERAL 111
  LITERAL 110
MAX_REPEAT 0 1
  IN
    CATEGORY CATEGORY_SPACE
LITERAL 101
LITERAL 116
LITERAL 99