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]

# python 3

# regex matching email email address

import re

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

if xx:
    print("matched")
    print(xx.group(1))
else:
    print("no match")

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 example of using regex to replace text.

# python 3

# example of regex replace

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.

# python 3

# regex example of replacing gif to png in html img tag

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)

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

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

  1. Python 3 Basics
  2. Python 2 Basics
  3. Python 2 and 3 Difference
  4. Print Version
  5. Builtin Help
  6. Quote String
  7. String Methods
  8. Format String
  9. Operators
  10. Complex Numbers
  11. True, False
  12. if then else
  13. Loop
  14. List Basics
  15. Loop Thru List
  16. Map f to List
  17. Copy Nested List
  18. List Comprehension
  19. List Methods
  20. Sort
  21. Dictionary
  22. Loop Thru Dict
  23. Dict Methods
  24. Tuple
  25. Sets
  26. Function
  27. Closure
  28. 2 Closure
  29. Decorator
  30. Class
  31. Object, ID, Type
  32. List Modules
  33. Write a Module
  34. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. File 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

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Append String in Loop
  7. Timing f timeit
  8. Keyword Arg Default Value Unstable
  9. Check Page Load Size
  10. Thumbnail Generation