You can use the re module in Python to find matching characters in a string using regular expressions.
The re.findall() function can be used to find all occurrences of a pattern in a string.
For example, the following code finds all occurrences of the letter 'a' in a string:
import re
string = "The quick brown fox jumps over the lazy dog."
match = re.findall(r'a', string)
print(match)
This would return the following output:
['a', 'a', 'a']
You can also use re.search() to find the first match of a pattern in a string,
or re.finditer() to return an iterator yielding match objects.