Python: Regex Basics

By Xah Lee. Date: . Last updated: .

Check If String Match

To use regex in Python, first you need to import re.

To check if a pattern is in string, use:

re.search(pattern, str, flags) → If pattern matches (part or whole of a string), then a Match Object is returned. Else, Returns None. (Match Object evaluates to True) [see Python Regex Match Object]

# -*- coding: utf-8 -*-
# python

# simple example of finding email address

import re

xx = re.search(r" (\w+@\w+\.com) ", "this xyz@example.com that")

if xx:
    print xx.group(1) # → xyz@xyz.com
else:
    print "no"

For how to use regex flags, such as ignore case, see: Python Regex Flags.

Find and Replace

sub(pattern, repl, string) → Substitute pattern in string by the replacement repl. If the pattern isn't found, string is returned unchanged. Returns a new string.

Here's a simple example of using regex to replace text.

#-*- coding: utf-8 -*-
# python

# simple example of using regex to replace text

import re

myText = "123123";

newText = re.sub(r"2", r"8", myText)

print newText; # 183183

Here's a more complex example, replacing all “gif” image paths to “png” in HTML file.

# -*- coding: utf-8 -*-
# python

import re

myText = r"""<p><img src="./rabbits.gif" width="30" height="20">
and <img class="xyz" src="../cats.gif">,
but <img src ="tigers.gif">,
 <img src=
"bird.gif">!</p>"""

newText = re.sub(r'src\s*=\s*"([^"]+)\.gif"', r'src="\1.png"', myText)

print newText

You should use r to quote your regex pattern string, so any backslash such as are not interpreted. [see Python: Quote String]

Split Line by Regex

Python: Split Line by Regex

Python Regex Reference

Python Regex Reference

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Python by Example

  1. Python Basics
  2. Print Version String
  3. Builtin Help
  4. Quote String
  5. String Operations
  6. String Methods
  7. Format String
  8. True, False
  9. if then else
  10. for, while, Loops
  11. List Basics
  12. Loop Thru List
  13. Map Function to List
  14. List Comprehension
  15. List Methods
  16. Dictionary
  17. Loop Thru Dict
  18. Dict Methods
  19. Function
  20. Class
  21. List Modules
  22. Write a Module
  23. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. Manipulate Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency

Web

  1. Send Email
  2. GET Web Page
  3. Web Crawler
  4. HTTP POST
  5. Check Page Load Size
  6. Thumbnail Generation

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Complex Numbers

Advanced

  1. Sort
  2. Copy Nested List
  3. Tuple vs List
  4. Sets, Union, Intersection
  5. Closure in Python 2
  6. Decorator
  7. Append String in Loop
  8. Timing f timeit
  9. Keyword Arg Default Value Unstable