How to sub first character in a text file with regex in python?

@aubrey 

You can achieve this by reading the text file, applying a regex pattern to find the first character, and then substituting it with the desired character. Here's an example code snippet in Python:

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import re

# Read the text file
with open('input.txt', 'r') as file:
    text = file.read()

# Define the regex pattern to find the first character
pattern = re.compile(r'^.')

# Substitute the first character with a new character
new_text = re.sub(pattern, 'X', text, count=1)

# Write the updated text back to the file
with open('output.txt', 'w') as file:
    file.write(new_text)

In this code snippet, we first read the content of the input text file. We then define a regex pattern that matches the first character in the text. We use the re.sub function to substitute the first character with a new character 'X'. Finally, we write the updated text back to an output text file.

Make sure to customize the file paths and the substitute character according to your requirements.