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.
Import the re module:import re
RegEx in Python
When you have imported the re module, you can start using regular 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)
Output:
YES! We have a match!
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 |
The findall() Function
The findall() function returns a list containing all matches.
Example
Print a list of all matches:
import re
#Return a list containing every occurrence of “ai”:
txt = “The rain in Spain”
x = re.findall(“ai”, txt)
print(x)
Output:
[‘ai’, ‘ai’]
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned:
Example
Return an empty list if no match was found:
import re
txt = “The rain in Spain”
#Check if “Portugal” is in the string:
x = re.findall(“Portugal”, txt)
print(x)
if (x):
print(“Yes, there is at least one match!”)
else:
print(“No match”)
Output:
[]
No match
The search() Function
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:
Example
Search for the first white-space character in the string:
import re
txt = “The rain in Spain”
x = re.search(“\s”, txt)
print(“The first white-space character is located in position:”, x.start())
Output:
The first white-space character is located in position: 3
If no matches are found, the value None is returned:
Example
Make a search that returns no match:
import re
txt = “The rain in Spain”
x = re.search(“Portugal”, txt)
print(x)
Output:
None
The split() Function
The split() function returns a list where the string has been split at each match:
Example
Split at each white-space character:
import re
txt = “The rain in Spain”
x = re.split(“\s”, txt)
print(x)
Output:
[‘The’, ‘rain’, ‘in’, ‘Spain’]
You can control the number of occurrences by specifying the maxsplit parameter:
Example
Split the string only at the first occurrence:
import re
txt = “The rain in Spain”
x = re.split(“\s”, txt, 1)
print(x)
Output:
[‘The’, ‘rain in Spain’]
The sub() Function
The sub() function replaces the matches 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)
print(x)
Output:
The9rain9in9Spain
You can control the number of replacements by specifying the count parameter:
Example
Replace the first 2 occurrences:
import re
txt = “The rain in Spain”
x = re.sub(“\s”, “9”, txt, 2)
print(x)
Output:
The9rain9in Spain
Metacharacters
Example
import re
txt = “The rain in Spain”
#Find all lower case characters alphabetically between “a” and “m”:
x = re.findall(“[a-m]”, txt)
print(x)
Output:
[‘h’, ‘e’, ‘a’, ‘i’, ‘i’, ‘a’, ‘i’]
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 | “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
Example
import re
txt = “The rain in Spain”
#Check if the string starts with “The”:
x = re.findall(“\AThe”, txt)
print(x)
if x:
print(“Yes, there is a match!”)
else:
print(“No match”)
Output:
[‘The’]
Yes, there is a match!
A special sequence is a \ 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
Example
import re
txt = “The rain in Spain”
#Check if the string has any a, r, or n characters:
x = re.findall(“[arn]”, txt)
print(x)
if x:
print(“Yes, there is at least one match!”)
else:
print(“No match”)
Output:
[‘r’, ‘a’, ‘n’, ‘n’, ‘a’, ‘n’]
Yes, there is at least one match!
A set is a set of characters inside a pair of square brackets [] with a 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 |
Match Object
A Match Object is an object containing information about the search and the result.
Note: If there is no match, the value None will be returned, instead of the Match Object.
Example
Do a search that will return a Match Object:
import re
txt = “The rain in Spain”
x = re.search(“ai”, txt)
print(x) #this will print an object
Output:
<_sre.SRE_Match object; span=(5, 7), match=’ai’>
The Match object has properties and methods used to retrieve 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
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.span())
Output:
(12, 17)
Example
Print the string passed into the function:
import re
txt = “The rain in Spain”
x = re.search(r”\bS\w+”, txt)
print(x.string)
Output:
The rain in Spain
Example
Print the part of the string where there was a match.
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.group())
Output:
Spain
Note: If there is no match, the value None will be returned, instead of the Match Object.