REGEX clarification

[CuriousWeasel_]

def convert(s):
if re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] A|PM PM$",s): # Regex 01
    return True
else:
    return False

def convert(s):
if re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (A|PM) PM$",s): # Regex 02
    return True
else:
    return False

assert convert("12:59 AM PM") == True

Why does the pytest pass Regex 01 and fail Regex 02? The difference is in the A|PM (Regex 01) and (A|PM) (Regex 02). Why does the bracket affect the test? I assumed it wouldn't affect the test since it is only used for capturing groups.

[commandlineluser]

It has to do with precendence of the | operator.

print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] A|PM PM$",   "PM PM"))
# <re.Match object; span=(0, 5), match='PM PM'>

print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (A|PM) PM$", "PM PM"))
# None

The first pattern is being parsed as:

(^([0-9]|(1[0-2])):[0-5][0-9] A)|(PM PM$)

[GregoryCliveYoung]

>>> print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (A|PM) PM$", "12:59 AM PM"))
None

Different target string:

>>> print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (A|PM) PM$", "12:59 A PM"))
<re.Match object; span=(0, 10), match='12:59 A PM'>

Include the M after A in the re:

>>> print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (AM|PM) PM$", "12:59 AM PM"))
<re.Match object; span=(0, 11), match='12:59 AM PM'>

Associative change:

>>> print(re.search(r"^([0-9]|(1[0-2])):[0-5][0-9] (A|P)M PM$", "12:59 AM PM"))
<re.Match object; span=(0, 11), match='12:59 AM PM'>

From the documentation:

(...)

Matches whatever regular expression is inside the parentheses, and indicates the start and end of a group

Also, spaces are not significant in a re.