Regex

Basics

Quantifier

Regular Expression	Description
*	Occurs zero or more times, is short for {0,}
+	Occurs one or more times, is short for {1,}
?	Occurs no or one times, ? is short for {0,1}
{X}	Occurs X number of times, {} describes the order of the preceding liberal
{X,Y}	Occurs between X and Y times
*?	? after a quantifier makes it a reluctant quantifier. It tries to find the smallest match. This makes the regular expression stop at the first match.

Meta characters

Regular Expression	Description
\d	Any digit, short for [0-9]
\D	A non-digit, short for
\s	A whitespace character, short for [ \t\n\x0b\r\f]
\S	A non-whitespace character, short for
\w	A word character, short for [a-zA-Z_0-9]
\W	A non-word character
\b	Matches a word boundary where a word character is [a-zA-Z0-9_]

Look around

Lookaround	Name	What it Does
(?=foo)	Lookahead	Asserts that what immediately FOLLOWS the current position in the string is foo
(?<=foo)	Lookbehind	Asserts that what immediately PRECEDES the current position in the string is foo
(?!foo)	Negative Lookahead	Asserts that what immediately FOLLOWS the current position in the string is NOT foo
(?<!foo)	Negative Lookbehind	Asserts that what immediately PRECEDES the current position in the string is NOT foo

Python

import re

Substitution

s = re.sub(r"([.!?])", r" \1", s)

The first \1 means the first group - i.e. the first bracketed expression ([.!?])

From the docs \number

References

1. Regular expressions in Java - Tutorial

2. Stack Overflow: How to match, but not capture, part of a regex?