add regex to api python code example

Example 1: regex findall

import re
# regex for finding mentions in a tweet
regex = r"(?<!RT\s)@\S+"
tweet = '@tony I am so over @got and @sarah is dead to me.'

# mentions = ['@tony', '@got', '@sarah'] 
mentions = re.findall(regex, tweet)

Example 2: python re compile

import re
#	Compile a regular expression pattern into a regular expression object, which can be used for matching using its match(), search() and other methods, described below.

prog = re.compile(pattern)
result = prog.match(string)

#	is equivalent to

result = re.match(pattern, string)