about pattern pattern = r"(.+) \1" and pattern = r"(.+) \2"

Question

Hi Dear,
Can you please explain these pattern for me? pattern = r"(.+) \1" and pattern = r"(.+) \2"

when i use the following script, there is no problem:
import re
pattern = r"(.+) \1"
match = re.match(pattern, "word word")
if match:
print ("Match 1")

but when i change the pattern to r"(.+) \2" it rise an error. please explain exactly what is this pattern mean.

import re
pattern = r"(.+) \2"
match = re.match(pattern, "egg egg egg")
if match:
print ("Match 1")

Answer 1

In Regular expression, the '\1' references the first capturing group. And the '\2' referencees the second capturing group.

Forexample,
pattern = r"(.+) (.+) \1 \2"
match = re.match(pattern, "egg1 egg2 egg1 egg2")

In this code, the first '(.+)' is the first capturing group.
the second '(.+)' is the second capturing group.

When this code is executed, it matches as follows

The first (.+) matches the first 'egg1'. therefore, the '\1' matches second 'egg1'.

The second (.+) matches the first 'egg2'. And, the '\2' matches the second 'egg2'

In your code, You can not use '\2' because there is only one '(.+)'.