正则表达式 regular expression

import re

Match Object

A Match Object is an object containing information about the search and the result.

.span() returns a tuple containing the start-, and end positions of the match. .string returns the string passed into the function .group() returns the part of the string where there was a match

#Search for an upper case "S" character in the beginning of a word
txt = "The S rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())  # (14, 19)
print(x.string)  # The S rain in Spain
print(x.group())  # Spain

Functions

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

findall

The findall() function returns a list containing all matches.

txt = "The rain in Spain"
x = re.findall("^ai", txt)  # []
x = re.findall("ai", txt)  # ['ai', 'ai']
x = re.findall(r".i", txt)  # ['ai', ' i', 'ai']

search

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

txt = "The rain in Spain"
x = re.search("\s", txt)  # <re.Match object; span=(3, 4), match=' '>
x = re.search("ai", txt)  # <re.Match object; span=(5, 7), match='ai'>
x = re.search("aai", txt)  # None

split

The split() function returns a list where the string has been split at each match:

txt = "The rain in Spain"
x = re.split("\s", txt)  # ['The', 'rain', 'in', 'Spain']
x = re.split("aai", txt)  # ['The rain in Spain']

sub

The sub() function replaces the matches with the text of your choice:

txt = "The rain in Spain"
x = re.sub("\s", "_", txt)  # The_rain_in_Spain
x = re.sub("aai", "_", txt)  # The rain in Spain
x = re.sub("\s", "_", txt, count=2)  # The_rain_in Spain

Metacharacters

Character	Description	Example
[]	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	"planet$"
*	Zero or more occurrences	"he.*o"
+	One or more occurrences	"he.+o"
?	Zero or one occurrences	"he.?o"
{}	Exactly the specified number of occurrences	"he.{2}o"
|	Either or	"falls|stays"
()	Capture and group

Special Sequences

Character	Description	Example
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"
\b	Returns a match where the specified characters are at the beginning or at the end of a word	r"\bain" r"ain\b"
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word	r"\Bain" r"ain\B"
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"
\D	Returns a match where the string DOES NOT contain digits	"\D"
\s	Returns a match where the string contains a white space character	"\s"
\S	Returns a match where the string DOES NOT contain a white space character	"\S"
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	"\w"
\W	Returns a match where the string DOES NOT contain any word characters	"\W"
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"

Sets

Set	Description
[arn]	Returns a match where one of the specified characters (a, r, or n) is 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