To replace the first two characters of a string with 'Hello':

x = "This is a test"
puts x.sub(/^../, 'Hello')

1.	Basic Special Characters and Symbols Within Regular Expressions
2.	Regular Expression Character and Sub-Expression Modifiers
3.	To change the last two letters
4.	scan is the iterator method you require:
5.	scan(/../): . means Any character
6.	scan (/\w\w/): \w means Any letter, digit, or underscore
7.	Index string by regular expression

To replace the first two characters of a string with 'Hello': : sub with regular expressions « String « Ruby