ref: Python RegEx
A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.
RegEx can be used to check if a string contains the specified search pattern.
RegEx Module
Python has a built-in package called re, which can be used to work with Regular Expressions.
RegEx in Python
After importing the re module, we can start using regular expressions:
RegEx Functions
The re module offers a set of functions that allows us to search a string for a match:
| 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 |
Here are some exmaples:
a = "Today is sunny day and I am working now."
r1 = re.findall("\w ", a)
r2 = re.search("\w ", a)
r3 = re.split("\w ", a)
r4 = re.sub("\w ", "==", a)
print(r1)
print(r2)
print(r3)
print(r4)
# ['y ', 's ', 'y ', 'y ', 'd ', 'I ', 'm ', 'g ']
# <re.Match object; span=(4, 6), match='y '>
# ['Toda', 'i', 'sunn', 'da', 'an', '', 'a', 'workin', 'now.']
# Toda==i==sunn==da==an====a==workin==now.Metacharacters
Metacharacters are characters with a special meaning:
| Character | Description | Example | Try it |
|---|---|---|---|
| [] | A set of characters | "[a-m]" | Try it » |
| \ | Signals a special sequence (can also be used to escape special characters) | "\d" | Try it » |
| . | Any character (except newline character) | "he..o" | Try it » |
| ^ | Starts with | "^hello" | Try it » |
| $ | Ends with | "planet$" | Try it » |
| * | Zero or more occurrences | "he.*o" | Try it » |
| + | One or more occurrences | "he.+o" | Try it » |
| ? | Zero or one occurrences | "he.?o" | Try it » |
| {} | Exactly the specified number of occurrences | "he.{2}o" | Try it » |
| | | Either or | "falls|stays" | Try it » |
| () | 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:
| Character | Description | Example | Try it |
|---|---|---|---|
| \A | Returns a match if the specified characters are at the beginning of the string | "\AThe" | Try it » |
| \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" |
Try it » Try it » |
| \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" |
Try it » Try it » |
| \d | Returns a match where the string contains digits (numbers from 0-9) | "\d" | Try it » |
| \D | Returns a match where the string DOES NOT contain digits | "\D" | Try it » |
| \s | Returns a match where the string contains a white space character | "\s" | Try it » |
| \S | Returns a match where the string DOES NOT contain a white space character | "\S" | Try it » |
| \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" | Try it » |
| \W | Returns a match where the string DOES NOT contain any word characters | "\W" | Try it » |
| \Z | Returns a match if the specified characters are at the end of the string | "Spain\Z" | Try it » |
Sets
A set is a set of characters inside a pair of square brackets [] with a special meaning:
| Set | Description | Try it |
|---|---|---|
| [arn] | Returns a match where one of the specified characters (a, r, or n) is present |
Try it » |
| [a-n] | Returns a match for any lower case character, alphabetically between a and n |
Try it » |
| [^arn] | Returns a match for any character EXCEPT a, r, and n |
Try it » |
| [0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
Try it » |
| [0-9] | Returns a match for any digit between 0 and 9 |
Try it » |
| [0-5][0-9] | Returns a match for any two-digit numbers from 00 and 59 |
Try it » |
| [a-zA-Z] | Returns a match for any character alphabetically between a and z, lower case OR upper case |
Try it » |
| [+] | In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string |
Try it » |