Added the regex pypi package as an extras_require

.travis.yml

@@ -4,6 +4,8 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
 install:
   - pip install tox tox-travis
 script:

parsimonious/expressions.py

@@ -7,7 +7,11 @@
 # anything--for speed. And kill all the dots.
 
 from inspect import getargspec
-import re
+
+try:
+    import regex as re
+except ImportError:
+    import re
 
 from six import integer_types, python_2_unicode_compatible
 from six.moves import range

setup.py

@@ -26,6 +26,9 @@
     url='https://github.com/erikrose/parsimonious',
     include_package_data=True,
     install_requires=['six>=1.9.0'],
+    extras_require={
+        "regex": ["regex>=2020.04.04"],
+    },
     classifiers=[
         'Intended Audience :: Developers',
         'Natural Language :: English',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34, py35, py36
+envlist = py27, py34, py35, py36, py37, py38, py{27, 34, 35, 36, 37, 38}--regex
 
 [tox:travis]
 2.7 = py27
@@ -13,3 +13,4 @@ commands = nosetests parsimonious
 deps =
   nose
   six>=1.9.0
+  regex: regex