To replace the first two characters of a string with 'Hello':
x = "This is a test" puts x.sub(/^../, 'Hello')
Here, you make a single substitution with sub.
The first parameter given to sub isn't a string but a regular expression.
The forward slashes are used to start and end a regular expression.
Within the regular expression is ^...
The ^ is an anchor, meaning the regular expression will match from the beginning of any lines within the string.
The two periods each represent "any character."
/^../ means "any two characters immediately after the start of a line."
Therefore, Th of "This is a test" gets replaced with Hello.
To change the last two letters:
x = "This is a test" puts x.sub(/..$/, 'Hello')
The regular expression matches the two characters that are anchored to the end of any lines within the string.
To anchor to the absolute start or end of a string, use \A and \Z, respectively.
^ and $ anchor to the starts and ends of lines within a string.