Python re.sub changing parts of string into ascii

Question

test1 = "<test>222</test>blah blah blah"
newVal = "222"
mstring = "(<test>).*(</test>)"
newString = re.sub(mstring,rf"\1{newVal}\2",test1)
print(newString)

I am trying to find a particular value in my string and substitute with a different string using re.sub function. Seems like i am find the correct match and replace is working however python is converting parts of the string into its ascii equivalent value. Can you help me with the above code so that i produce the below output please

<test>222</test>blah blah blah

instead i am getting below result

R2</test>blah blah blah

@snakecharmerb I am using Python 3.7. I could try and upgrade to 3.8 and try again — Krishna, Nov 08 '19 at 16:27
@MarkRansom It can't work with your approach. The problem is given by ambiguity in replacement. See my answer for more details. — Riccardo Bucco, Nov 08 '19 at 16:35
@RiccardoBucco my mistake, I didn't see the `r` at the start of the string. I assumed the backslashes were being interpreted and not becoming part of the replacement string. — Mark Ransom, Nov 08 '19 at 16:53

Riccardo Bucco · Accepted Answer · 2019-11-08T16:46:27.373

2

Here is a possible solution:

test1 = "<test>222</test>blah blah blah"
newVal = "111"
mstring = "(<test>).*(</test>)"
newString = re.sub(mstring, f'\g<1>{newVal}\g<2>', test1)`
print(newString) # <test>111</test>blah blah blah

Your approach would work with newVal being, for example, a letter:

newVal = "a"
re.sub(mstring, f'\1{newVal}\2', test1) # <test>a</test>blah blah blah

This weird behaviour is due to the fact that \1{newVal} (with newVal=333) would be interpreted as a reference to group 1333. The \g<1> syntax is equivalent to \1, but isn’t ambiguous in a replacement.

edited Nov 08 '19 at 16:46

answered Nov 08 '19 at 16:18

Riccardo Bucco

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Thanks this is working for me – Krishna Nov 08 '19 at 16:28
1

@Krishna I updated the answer with more details explaining why your approach is (sometimes) failing – Riccardo Bucco Nov 08 '19 at 16:36
Thank you for the explanation. I started to learn python only recently but this looks like a very powerful functionality – Krishna Nov 08 '19 at 17:09

Python re.sub changing parts of string into ascii

1 Answers1