sub with regular expressions « String « Ruby

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