What is the difference between (? Imx) and (? Imx: re) in regular expressions?

What is the difference between (? Imx) and (? Imx: re) in regular expressions?

Can you give an example? I didn’t have any ideas


In the first form, the flags apply to the remainder of the pattern, whereas in the second form, the flags apply only to the contents of the parentheses:

# Case-insensitive 'a' and 'b'.
>>> re.match(r'(?i)ab', 'AB')
<re.Match object; span=(0, 2), match='AB'>
>>> re.match(r'(?i)ab', 'Ab')
<re.Match object; span=(0, 2), match='Ab'>

# Case-insensitive 'a' and case-sensitive 'b'.
>>> re.match(r'(?i:a)b', 'AB')
>>> re.match(r'(?i:a)b', 'Ab')
<re.Match object; span=(0, 2), match='Ab'>

And also remember that flags can be turned off as well as on, which is useful if, say, the pattern is mostly case-insensitive but with an case-insensitive part.

@yeshuo - The best way to explore this – after reading the docs at re — Regular expression operations — Python 3.11.5 documentation – is to open a command prompt, run python interactive and try things out.

>>> import re
>>> pat1 = re.compile(r"abc")
>>> pat2 = re.compile(r"(?imx)abc")
>>> pat1.match("ABC")
>>> pat2.match("ABC")
<re.Match object; span=(0, 3), match='ABC'>
>>> pat1.match("ABC") is None
>>> pat3 = re.compile(r"(?imx:re)abc")
>>> pat3.pattern
>>> pat3.match("ABC")  # nope
>>> pat3.match("abc")  # nope
>>> pat3.match("reabc")
<re.Match object; span=(0, 5), match='reabc'>
>>> pat3.match("REabc")
<re.Match object; span=(0, 5), match='REabc'>
>>> pat3.match("reABC")

The docs don’t give a (good) explanation of the purpose of this kind of syntax. The purpose is to provide inline regex compiler flags. So (?imx:pyth) means that the ‘pyth’ part will match without case-sensitivity (and the ‘m’ and ‘x’ really don’t make much sense, since that part can never be multi-line, and verbose syntax is also not used). This syntax makes it possible to match part of the target string case-insensitive and part case-sensitive, for instance.

I tried with another example using search It is syntax make for case-sensitive and case-insensitive

import re
regex = re.compile(r"hello")
match = regex.search("HELLO")
print(match) #None

regex = re.compile(r"HELLO")
match = regex.search("HELLO")
print(match) # <re.Match object; span=(0, 5), match='HELLO'>

regex = re.compile(r"(?imx: hello)")
match = regex.search("abchello")
print(match) # <re.Match object; span=(3, 8), match='hello'>

regex = re.compile(r"(?imx: hello)")
match = regex.search("hello")
print(match) # <re.Match object; span=(0, 5), match='hello'>