Metacharacters, Special Sequences, Sets in Python 3.7

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Metacharacters

Metacharacters are characters with a special meaning:

CharacterDescriptionExample
[]A set of characters"[a-m]"
\Signals a special sequence (can also be used to escape special characters)"\d"
.Any character (except newline character)"he..o"
^Starts with"^hello"
$Ends with"world$"
*Zero or more occurrences"aix*"
+One or more occurrences"aix+"
{}Exactly the specified number of occurrences"al{2}"
|Either or"falls|stays"
()Capture and group  

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

CharacterDescriptionExample
\AReturns a match if the specified characters are at the beginning of the string"\AThe"
\bReturns a match where the specified characters are at the beginning or at the end of a word
(the "r" in the beginning is making sure that the string is being treated as a "raw string")		r"\bain"
r"ain\b"
\BReturns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word
(the "r" in the beginning is making sure that the string is being treated as a "raw string")		r"\Bain"
r"ain\B"
\dReturns a match where the string contains digits (numbers from 0-9)"\d"
\DReturns a match where the string DOES NOT contain digits"\D"
\sReturns a match where the string contains a white space character"\s"
\SReturns a match where the string DOES NOT contain a white space character"\S"
\wReturns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)"\w"
\WReturns a match where the string DOES NOT contain any word characters"\W"
\ZReturns a match if the specified characters are at the end of the string"Spain\Z"

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

SetDescription
[arn]Returns a match where one of the specified characters (a, r, or n) are present
[a-n]Returns a match for any lower case character, alphabetically between a and n
[^arn]Returns a match for any character EXCEPT a, r, and n
[0123]Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9]Returns a match for any digit between 0 and 9
[0-5][0-9]Returns a match for any two-digit numbers from 00 and 59
[a-zA-Z]Returns a match for any character alphabetically between a and z, lower case OR upper case
[+]In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string

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