o=re.search(pattern,s)
pattern # the searched character
s: String is searched
o: # result object
The essence of the re module is the search function, that takes a regular expression (pattern) and a string ('M') to try to match against the regular expression. If a match is found, search returns an object which has various methods to describe the match; if no match is found, search returns None, the Python null value. All you care about at the moment is whether the pattern matches, which you can tell by just looking at the return value of search. 'M' matches this regular expression, because the first optional M matches and the second and third optional M characters are ignored.