Replacing four letter words in a string

Replacing four letter words in a string

References

Reference NO 1

http://stackoverflow.com/questions/15195976/replacing-four-letter-words-in-a-string

Reference NO 2

http://stackoverflow.com/questions/12723751/replacing-letter-in-string-by-index

Reference NO 3

http://www.dreamincode.net/forums/topic/119898-strings-finding-a-4-letter-word-in-a-string/

Reference NO 4

http://www.pythonforbeginners.com/strings/

Reference NO 5

http://cran.r-project.org/web/packages/stringr/stringr.pdf

Reference NO 6

http://en.wikibooks.org/wiki/Python_Programming/Strings

Reference NO 7

http://www.learnpython.org/Basic_String_Operations

Reference NO 8

http://www.ats.ucla.edu/stat/stata/faq/regex.htm