Python RegEx

In Python, RegEx is short for regular expression. RegEx is a sequence of characters that defines a search pattern. It is used to search for and manipulate text strings based on patterns, rather than exact matches of specific characters.

Python has built-in support for regex through the re module.

The re module in Python provides support for regular expressions. It includes several functions and methods that allow you to search and manipulate text strings using regular expressions. Some of the most commonly used functions include search(), compile(), findall(), split(), and sub().

Here is an example of how to use regex in Python:

import re

text = "Learn to code by reading and understanding the concept."
pattern = r"code"

# Search for the word "code" in the text
match = re.search(pattern, text)

if match:
    print("Found the word:", match.group())
else:
    print("Not found")

Output

In this example, we first start by importing the re module. The re.search() function is used to search for the string "code" in the text variable. The r before the string indicates that it is a raw string, which is used to avoid having to escape backslashes in the string.

If the pattern is found, the search() function returns a match object, which can be used to extract the matched substring. The group() method is used to extract the matched substring.

Here is another example to search for the words "code is 9091" from a given text:

import re

text = "It is a sequence of numbers from 0 to 9. The code is 9091."
pattern = r"\bcode is \d+\b"

result = re.search(pattern, text)
if result:
    print("Found: ", result.group(0))
else:
    print("Not found") 

Output

In this example, \b in the pattern matches the specified pattern at the beginning or at the end of a word. The \d+ pattern matches one or more numeric digit in a string.

Here are a few more examples with complex pattern matching:

import re

text = "apple is a fruit. The boy is eating apple. sun rise in the east."

# pattern to search for words that have "e" as the second letter
pattern1 = r"\b\w[p]\w*\b"

# pattern to search for words that end with "ing"
pattern2 = r"\w+ing\b"

# pattern to search for words that start with "s"
pattern3 = r"\bs\w*"


match = re.search(pattern1, text)
print(match.group())

match = re.search(pattern2, text)
print(match.group())

match = re.search(pattern3, text)
print(match.group())

Output

The search() Function

The search(pattern, string, flags=0) function scans a string looking for a match to the pattern and returns a Match object if match is found or None if no match is found.

Example

import re

text = "The code is 9097"
pattern = r"\d+"

result = re.search(pattern, text)
print(result.group())

The compile() Function

The compile(pattern, repl, string) function compiles a regular expression and return a Pattern object.

Example

import re

text = "The code is 9091. The code is 9091."
pattern = "9091"
pattern_obj = re.compile(pattern)
print(pattern_obj.findall(text))

Output

The findall() Function

The findall(pattern, string, flags=0) function returns a list of all non-overlapping matches of pattern in string.

Example

import re

text = "It is a sequence of numbers from 0 to 9. The code is 9091."
pattern = "\d+"
result = re.findall(pattern,text)
print(result)

The split() Function

The split(pattern, string, maxsplit=0, flags=0) function splits the source string at each match of the pattern and returns the resulting list of substrings.

Example

import re

text = "mango1apple2grapes3orange4pear"
pattern = "\d+"
result = re.split(pattern,text)
print(result)

The sub() Function

The sub(pattern, repl, string, count=0, flags=0) function replaces the leftmost non-overlapping occurrences of the pattern in string and returns the resulting string. The unchanged string is returned if the pattern is not found.

Example

import re

text = "mango1apple2grapes3orange4pear"
pattern = "\d+"
repl = " "
result = re.sub(pattern,repl,text)
print(result)

RegEx Metacharacters

Metacharacters are characters with special meaning. Following are the list of RegEx metacharacters in Python:

RegEx Sequence Characters

RegEx uses the \ character to allow special characters to be used without invoking their special meaning. Following are the list of RegEx Backslash Characters in Python: