[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Renji]

Renji  added the comment:

I through "forward reference" is "\1 (abcd)". Not "some sort of reference in 
second repetition to data from first repetition".

Ok. In other words refers from on repetition to other supported, but with 
purely formal restrictions. And remove this restrictions don't planned. Than 
this issue may be closed.

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[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Matthew Barnett]

Matthew Barnett  added the comment:

Example 1:

((a)|b\2)*
 ^^^   Group 2

((a)|b\2)*
  ^^   Reference to group 2

The reference refers backwards to the group.

Example 2:

(b\2|(a))*
 ^^^   Group 2

(b\2|(a))*
  ^^   Reference to group 2

The reference refers forwards to the group.

As I said, the re module doesn't support forward references to groups.

If you have a regex where forward references are unavoidable, try the 3rd-party 
'regex' module instead. It's available on PyPI.

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[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Renji]

Renji  added the comment:

In my example reference and capture group presents in two difference 
alternatives. They don't follow each other, but executed in random order. If 
this don't supported in one case, why it supported in other case?

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[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Matthew Barnett]

Matthew Barnett  added the comment:

It's not a crash. It's complaining that you're referring to group 2 before 
defining it. The re module doesn't support forward references to groups, but 
only backward references to them.

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[issue42871] Regex compilation crashed if I change order of alternatives under quantifier

[Renji]

New submission from Renji :

I can compile "((a)|b\2)*" expression and this expression successfully return 
captures from first repetition and second repetition in one time. But if I 
write (b\2|(a))* expression, I get "invalid group reference 2 at position 3" 
error. Either first or second behavior incorrect.
python3 --version Python 3.7.3

import re
text="aba"
#match=re.search(r"(b\2|(a))*",text) - not worked
match=re.search(r"((a)|b\2)*",text)
if(match):
#show aba ba a
print(match.group(0)+" "+match.group(1)+" "+match.group(2))

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components: Regular Expressions
messages: 384703
nosy: Renji, ezio.melotti, mrabarnett
priority: normal
severity: normal
status: open
title: Regex compilation crashed if I change order of alternatives under 
quantifier
type: behavior
versions: Python 3.7

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