import re


def casi(inp):
    """Make string for case insenitive regular expression string::

>>> casi('abc') == '[aA][bB][cC]'
True
"""
    outp = []
    letter = re.compile(r'^\w$')
    whitespace = re.compile(r'^\s$')
    for char in inp:
        if letter.match(char):
            outp.extend(('[', char.lower(), char.upper(), ']'))
        elif whitespace.match(char):
            outp.append('\\s')
        else:
            outp.append(char)
    return ''.join(outp)