Regular expressions - match only specified string length
Ref. http://www.regular-expressions.info/anchors.html


  Anchors are a different breed. They do not match any character at all. Instead, they match a position before, after, or between characters. They can be used to "anchor" the regex match at a certain position. The caret ^ matches the position before the first character in the string. Applying ^a to abc matches a. ^b does not match abc at all, because the b cannot be matched right after the start of the string, matched by ^. See below for the inside view of the regex engine.
  
  Similarly, $ matches right after the last character in the string. c$
  matches c in abc, while a$ does not match at all.


With anchors you may skip length test entirely.

So:

import re
str = raw_input("input in form 'aA1' please &gt;")
check = re.compile("^[a-z][A-Z][0-9]$")
validate = check.match(str)
if validate:
    print "Correct string"
else:
    print "Incorrect string"