The reported behavior is reproducible in master as well as of ea68d83933 but not on 3.6.0. I couldn't bisect to the exact commit between 3.7.0 and 3.6.0 where this change was introduced though. I can also see some deprecation warnings as below while running the script : 

➜  cpython git:(master) ./python.exe ../backups/bpo34034.py
../backups/bpo34034.py:4: DeprecationWarning: invalid escape sequence \[
  sym = ['\[','\]','\(','\)']
../backups/bpo34034.py:4: DeprecationWarning: invalid escape sequence \]
  sym = ['\[','\]','\(','\)']
../backups/bpo34034.py:4: DeprecationWarning: invalid escape sequence \(
  sym = ['\[','\]','\(','\)']
../backups/bpo34034.py:4: DeprecationWarning: invalid escape sequence \)
  sym = ['\[','\]','\(','\)']
../backups/bpo34034.py:15: DeprecationWarning: invalid escape sequence \(
  result=re.match("DBMS_NAME python4: string\(\d+\) .*?", "DBMS_NAME python4: string(8) \"DB2/NT64\" ")
DBMS_NAME: string(%d) %s
DBMS_NAME: string(%d) %s
DBMS_NAME: string\(%d) %s
DBMS_NAME: string\(%d\) %s
DBMS_NAME: string\(%d\) .*?
Traceback (most recent call last):
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/sre_parse.py", line 1045, in parse_template
    this = chr(ESCAPES[this][1])
KeyError: '\\d'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "../backups/bpo34034.py", line 11, in <module>
    pattern = re.sub('%d', '\\d+', pattern)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/re.py", line 192, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/re.py", line 309, in _subx
    template = _compile_repl(template, pattern)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/re.py", line 300, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/sre_parse.py", line 1048, in parse_template
    raise s.error('bad escape %s' % this, len(this))
re.error: bad escape \d at position 0


Thanks