re.escape() escapes all the characters except ASCII letters, numbers and '_'. This is too excessive, makes escaping and compiling slower and makes the pattern less human-readable. Characters "!\"%&\',/:;<=>@_`~" as well as non-ASCII characters are always literal in a regular expression and don't need escaping.

Proposed patch makes re.escape() escaping only minimal set of characters that can have special meaning in regular expressions. This includes special characters ".\\[]{}()*+?^$|", "-" (a range in a character set), "#" (starts a comment in verbose mode) and ASCII whitespaces (ignored in verbose mode).

The null character no longer need a special escaping.

The patch also increases the speed of re.escape() (even if it produces the same result).

$ ./python -m perf timeit -s 'from re import escape; s = "()[]{}?*+-|^$\\.# \t\n\r\v\f"' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 42.2 us +- 0.8 us
Patched:    Median +- std dev: 11.4 us +- 0.1 us

$ ./python -m perf timeit -s 'from re import escape; s = b"()[]{}?*+-|^$\\.# \t\n\r\v\f"' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 38.7 us +- 0.7 us
Patched:    Median +- std dev: 18.4 us +- 0.2 us

$ ./python -m perf timeit -s 'from re import escape; s = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 40.3 us +- 0.5 us
Patched:    Median +- std dev: 33.1 us +- 0.6 us

$ ./python -m perf timeit -s 'from re import escape; s = b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 54.4 us +- 0.7 us
Patched:    Median +- std dev: 40.6 us +- 0.5 us

$ ./python -m perf timeit -s 'from re import escape; s = "абвгґдеєжзиіїйклмнопрстуфхцчшщьюяАБВГҐДЕЄЖЗИІЇЙКЛМНОПРСТУФХЦЧШЩЬЮЯ"' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 156 us +- 3 us
Patched:    Median +- std dev: 43.5 us +- 0.5 us

$ ./python -m perf timeit -s 'from re import escape; s = "абвгґдеєжзиіїйклмнопрстуфхцчшщьюяАБВГҐДЕЄЖЗИІЇЙКЛМНОПРСТУФХЦЧШЩЬЮЯ".encode()' -- --duplicate 100 'escape(s)'
Unpatched:  Median +- std dev: 200 us +- 4 us
Patched:    Median +- std dev: 77.0 us +- 0.6 us

And the speed of compilation of escaped string.

$ ./python -m perf timeit -s 'from re import escape; from sre_compile import compile; s = "абвгґдеєжзиіїйклмнопрстуфхцчшщьюяАБВГҐДЕЄЖЗИІЇЙКЛМНОПРСТУФХЦЧШЩЬЮЯ"; p = escape(s)' -- --duplicate 100 'compile(p)'
Unpatched:  Median +- std dev: 1.96 ms +- 0.02 ms
Patched:    Median +- std dev: 1.16 ms +- 0.02 ms

$ ./python -m perf timeit -s 'from re import escape; from sre_compile import compile; s = "абвгґдеєжзиіїйклмнопрстуфхцчшщьюяАБВГҐДЕЄЖЗИІЇЙКЛМНОПРСТУФХЦЧШЩЬЮЯ".encode(); p = escape(s)' -- --duplicate 100 'compile(p)'
Unpatched:  Median +- std dev: 3.69 ms +- 0.04 ms
Patched:    Median +- std dev: 2.13 ms +- 0.03 ms

New changeset 5908300e4b0891fc5ab8bd24fba8fac72012eaa7 by Serhiy Storchaka in branch 'master':
bpo-29995: re.escape() now escapes only special characters. (#1007)
https://github.com/python/cpython/commit/5908300e4b0891fc5ab8bd24fba8fac72012eaa7

New changeset a895f91a46c65a6076e8c6a28af0df1a07ed60a2 by terryjreedy in branch '3.6':
[3.6]bpo-29995: Adjust IDLE test for 3.7 re.escape change [GH-1007] (#2114)
https://github.com/python/cpython/commit/a895f91a46c65a6076e8c6a28af0df1a07ed60a2

Serhiy, please nosy me when you change idlelib files.

Aaaand this broke my unit tests when moving from 3.6 to 3.7!