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

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1

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

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

Thanks

2

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.

3

@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
True
>>> pat3 = re.compile(r"(?imx:re)abc")
>>> pat3.pattern
'(?imx:re)abc'
>>> 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.

4

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'>