Metacharacters
[] : A set of characters
\ : Signals a special sequence (can also be used to escape special characters) "\d"
. : Single dot to match 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 "x|y" matches either "x" or "y"
() : Capture and group
Special Sequences
\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
(the "r" in the beginning is making sure that the string is being treated as a "raw string") 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
(the "r" in the beginning is making sure that the string is being treated as a "raw string") 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 or Character Class
[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
Python regular expression
The re module in Python provides functions and support for regular expressions.
re.search()
re.match()
re.sub()
Replace "xxx" or "yyy" with "abc"
>>> old_string = "I xxx Python yyy"
>>> new_string = re.sub(r"xxx|yyy", "abc", old_string)
Replace first "xxx" with "abc"
>>> old_string = "abc xxx abc xxx"
>>> new_string = re.sub("xxx", "abc", old_string, 1)
>>> print(new_string)
