python regular expression sub only one position code example

Example 1: python replace regex

import re
s = "Example String"
replaced = re.sub('[ES]', 'a', s)
print replaced 
# will print 'axample atring'

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)