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Python RegEx
RegEx, or Regular Expression, is a sequence of characters that form a search pattern.
RegEx can be used to check if a character unit contains a specified search pattern.
RegEx Module
Python has a built-in package called re, which can be used to work with Regular Expressions.
Import the re module:
import re
RegEx in Python
Once you re the module, you can start using standard expressions:
Example
Search the string to see if it starts with "The" and ends with "Spain":
import
re
txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
RegEx Functions
The re module provides a set of functions that allow us to search for a match string:
| 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 |
Metacharacters
Metacharacters are characters with a special meaning:
| 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
The special sequence is \ followed by one of the characters in the list below, and has a special meaning:
| 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 (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
A set of characters within a pair of square brackets [] with special meaning:
| Set | Description |
|---|---|
| [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 |
The findall() Function
The findall() function returns a list containing all the same.
Example
Print a list of all matches:
import re
txt = "The rain in Spain"
x = re.findall("Portugal",
txt)
print(x)
The list contains the same as how it is obtained.
If no match was found, the blank list is returned:
Example
Return an empty list if no match was found:
import re
txt = "The rain in Spain"
x = re.search("\s",
txt)
print("The first white-space character is located in
position:", x.start())
The search() Function
The search() function searches for a thread to find the match, then returns the Match item if there is a match.
If there is more than one match, only the first action of the game will be returned:
Example
Search for the first white-space character in the string:
import re
txt = "The rain in Spain"
x = re.search("Portugal",
txt)
print(x)
If no match was found, the None is returned:
Example
Make a search that returns no match:
import re
txt = "The rain in Spain"
x = re.split("\s",
txt)
print(x)
The split() Function
The split() function returns the list where the character unit is separated for each game:
Example
Split at each white-space character:
import re
txt = "The rain in Spain"
x = re.split("\s",
txt,
1)
print(x)
You can control the number of occurrences by specifying a maxsplit parameter:
Example
Split the string only at the first occurrence:
import re
txt = "The rain in Spain"
x = re.sub("\s",
"9", txt)
print(x)
the sub() Function
The sub() function replaces the match with the text of your choice:
Example
Replace every white-space character with the number 9:
import re
txt = "The rain in Spain"
x = re.sub("\s",
"9", txt, 2)
print(x)
You can control the amount of changes by specifying a count parameter:
Example
Replace the first 2 occurrences:
import re
txt = "The rain in Spain"
x = re.search("ai",
txt)
print(x) #this will print an object
Match Object
Match Object is an item that contains information about the search and the result.
Note: If no match, None will be returned, instead of the Same Object.
Example
Do a search that will return a Match Object:
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())
The object object has features and methods used to retrieve information about the search, as well as the result:
.span() returns the tuple containing the first, last place for a game.
.string() returns the unit of characters that was transferred to the function
.group() returns the part of the character unit where it was the same
Example
Print the position (start- and end-position) of the first match occurrence.
The regular expression looks for any words that starts with an upper case "S":
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string)
Example
Print the string passed into the function:
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group())
Note: If no match, None will be returned, instead of the Same Object.