Python 3 - Regular Expressions - A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pat. .NET, Java, Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore. Unlike lots of other cheat sheets or regex web sites, I was. Regex Cheat Sheet The original cheat sheet can be found here https. Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore. Real Python: Python 3 Cheat Sheet. Contents 1 Introduction 2. Python is a beautiful language. It’s easy to learn and fun, and its syntax is simple yet ele.

By Alex Yang, Dataquest

This cheat sheet is based on Python 3’s documentation on regular expressions. If you're interested in learning Python, we have a free Python Programming: Beginner course for you to try out.

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 — Matches ASCII only
• i — Ignore case
• L — Locale dependent
• m — Multi-line
• s — Matches all
• u — Matches unicode
• x — 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 “name”.

(..)1 | The number 1 corresponds to the first group to be matched. If we want to match more instances of the same expresion, 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.

Useful Regular Expressions Sites for Python users

Bio: Alex Yang is a writer fascinated by the things code can do. He also enjoys citizen science and new media art.

Original. Reposted with permission.

Related:

• Python 3 Basic Tutorial

• Python 3 Advanced Tutorial

• Python 3 Useful Resources

• Selected Reading

A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. Regular expressions are widely used in UNIX world.

The module re provides full support for Perl-like regular expressions in Python. The re module raises the exception re.error if an error occurs while compiling or using a regular expression.

We would cover two important functions, which would be used to handle regular expressions. Nevertheless, a small thing first: There are various characters, which would have special meaning when they are used in regular expression. To avoid any confusion while dealing with regular expressions, we would use Raw Strings as r'expression'.

Basic patterns that match single chars

Compilation flags

Compilation flags let you modify some aspects of how regular expressions work. Flags are available in the re module under two names, a long name such as IGNORECASE and a short, one-letter form such as I.

The match Function

This function attempts to match RE pattern to string with optional flags.

Here is the syntax for this function −

Here is the description of the parameters −

The re.match function returns a match object on success, None on failure. We usegroup(num) or groups() function of match object to get matched expression.

Example

Python 3.8 Cheat Sheet

When the above code is executed, it produces the following result −

The search Function

This function searches for first occurrence of RE pattern within string with optional flags.

Python 3 Cheat Sheet Pdf

Here is the syntax for this function −

Here is the description of the parameters −

The re.search function returns a match object on success, none on failure. We use group(num) or groups() function of match object to get the matched expression.

Example

When the above code is executed, it produces the following result −

Matching Versus Searching

Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default).

Example

When the above code is executed, it produces the following result −

Search and Replace

One of the most important re methods that use regular expressions is sub.

Syntax

This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max is provided. This method returns modified string.

Example

When the above code is executed, it produces the following result −

Regular Expression Modifiers: Option Flags

Regular expression literals may include an optional modifier to control various aspects of matching. The modifiers are specified as an optional flag. You can provide multiple modifiers using exclusive OR (|), as shown previously and may be represented by one of these −

Regular Expression Patterns

Except for the control characters, (+ ? . * ^ $ ( ) [ ] { } | ), all characters match themselves. You can escape a control character by preceding it with a backslash.

The following table lists the regular expression syntax that is available in Python −

Regular Expression Examples

Literal characters

Character classes

Special Character Classes

Repetition Cases

Nongreedy repetition

This matches the smallest number of repetitions −

Grouping with Parentheses

Backreferences

This matches a previously matched group again −

Alternatives

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Anchors

Python 3 Regex Cheat Sheet Pdf

This needs to specify match position.

Python 3 Regex Cheat Sheet

Special Syntax with Parentheses

Python 3 Regex Cheat Sheet Answers