In Lib/idlelib/searchengine.py there is a code which decomposes re.error and reports error message, pattern, and position of error.

The problem is that that code does not match the structure of re.error. It takes args[0] as error message, and args[1] (if specified) as position in the pattern. But actually re.error always sets args[0] to formatted error message, and args has only one item. So some code in IDLE is dead.

Currently reported:

   Error: bad escape \z at position 1
   Pattern: a\z

Expected:

   Error: bad escape \z
   Pattern: a\z
   Offset: 1

The confusion about error.args is this:

BaseException.args is documented as "The tuple of arguments given to the exception constructor."
https://docs.python.org/3/library/exceptions.html#base-classes
args[0] is usually and I would expect it to always be the formatted message, which is to say, the string normally printed at the end of tracebacks.
The signature of re.error is documented as "(msg, pattern=None, pos=None)"
https://docs.python.org/3/library/re.html#re.error
and this is what inspect.signature says (as reported in IDLE calltip).
So one might expect .args to be a 3-tuple.

With the value fetching fixed, the displayed message could be changed to something else (in another issue), such as
  Error: bad escape \z in pattern r'a\z' at offset 1

Thanks for the patch.

New changeset 453bc1da2023d6cbe362637a2e0b06d0521f013c by Serhiy Storchaka in branch 'master':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
https://github.com/python/cpython/commit/453bc1da2023d6cbe362637a2e0b06d0521f013c

New changeset 28b40d7a349787817fe5f6ebfa753c29b96c8832 by Miss Islington (bot) in branch '3.8':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
https://github.com/python/cpython/commit/28b40d7a349787817fe5f6ebfa753c29b96c8832

New changeset dd20643b144d1c81989d235c3ec51f18fda618c2 by Miss Islington (bot) in branch '3.9':
bpo-42426: IDLE: Fix reporting offset of the RE error in searchengine (GH-23447)
https://github.com/python/cpython/commit/dd20643b144d1c81989d235c3ec51f18fda618c2