SPECIAL CHARACTERS
^ | Matches the expression to its right at the
start of a string. It matches every such
instance before each \n in the string.
$ | Matches the expression to its left at the
end of a string. It matches every such
instance before each \n in the string.
. | Matches any character except line
terminators like \n.
\ | Escapes special characters or denotes
character classes.
A|B | Matches expression A or B. If A is
matched first, B is left untried.
+ | Greedily matches the expression to its left 1
or more times.
* | Greedily matches the expression to its left
0 or more times.
? | Greedily matches the expression to its left
0 or 1 times. But if ? is added to qualifiers
(+, *, and ? itself) it will perform matches in
a non-greedy manner.
{m} | Matches the expression to its left m
times, and not less.
{m,n} | Matches the expression to its left m to
n times, and not less.
{m,n}? | Matches the expression to its left m
times, and ignores n. See ? above.
CHARACTER CLASSES
(A.K.A. SPECIAL SEQUENCES)
\w | Matches alphanumeric characters, which
means a-z, A-Z, and 0-9. It also matches
the underscore, _.
\d | Matches digits, which means 0-9.
\D | Matches any non-digits.
\s | Matches whitespace characters, which
include the \t, \n, \r, and space characters.
\S | Matches non-whitespace characters.
\b | Matches the boundary (or empty string)
at the start and end of a word, that is,
between \w and \W.
\B | Matches where \b does not, that is, the
boundary of \w characters.
\A | Matches the expression to its right at the
absolute start of a string whether in single
or multi-line mode.
\Z | Matches the expression to its left at the
absolute end of a string whether in single
or multi-line mode.
SETS
[ ] | Contains a set of characters to match.
[amk] | Matches either a, m, or k. It does not
match amk.
[a-z] | Matches any alphabet from a to z.
[a\-z] | Matches a, -, or z. It matches -
because \ escapes it.
[a-] | Matches a or -, because - is not being
used to indicate a series of characters.
[-a] | As above, matches a or -.
[a-z0-9] | Matches characters from a to z
and also from 0 to 9.
[(+*)] | Special characters become literal
inside a set, so this matches (, +, *, and ).
[^ab5] | Adding ^ excludes any character in
the set. Here, it matches characters that are
not a, b, or 5.
GROUPS
( ) | Matches the expression inside the
parentheses and groups it.
(?) | Inside parentheses like this, ? acts as an
extension notation. Its meaning depends on
the character immediately to its right.
(?PAB) | Matches the expression AB, and it
can be accessed with the group name.
(?aiLmsux) | Here, a, i, L, m, s, u, and x are
flags:
a โ Matches ASCII only
i โ Ignore case
L โ Locale dependent
m โ Multi-line
s โ Matches all
u โ Matches unicode
x โ Verbose
(?:A) | Matches the expression as represented
by A, but unlike (?PAB), it cannot be
retrieved afterwards.
(?#...) | A comment. Contents are for us to
read, not for matching.
A(?=B) | Lookahead assertion. This matches
the expression A only if it is followed by B.
A(?!B) | Negative lookahead assertion. This
matches the expression A only if it is not
followed by B.
(?<=B)A | Positive lookbehind assertion.
This matches the expression A only if B
is immediately to its left. This can only
matched fixed length expressions.
(?<!B)A | Negative lookbehind assertion.
This matches the expression A only if B is
not immediately to its left. This can only
matched fixed length expressions.
(?P=name) | Matches the expression matched
by an earlier group named โnameโ.
(...)\1 | The number 1 corresponds to
the first group to be matched. If we want
to match more instances of the same
expression, simply use its number instead of
writing out the whole expression again. We
can use from 1 up to 99 such groups and
their corresponding numbers.
POPULAR PYTHON RE MODULE
FUNCTIONS
re.findall(A, B) | Matches all instances
of an expression A in a string B and returns
them in a list.
re.search(A, B) | Matches the first instance
of an expression A in a string B, and returns
it as a re match object.
re.split(A, B) | Split a string B into a list
using the delimiter A.
re.sub(A, B, C) | Replace A with B in the
string C.
Data Science Cheat Sheet
Python Regular Expressions
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