Replace words in a string - Ruby

sentence.sub! 'Robert', 'Joe'

Won't cause an exception if the replaced word isn't in the sentence (the []= variant will).

How to replace all instances?

The above replaces only the first instance of "Robert".

To replace all instances use gsub/gsub! (ie. "global substitution"):

sentence.gsub! 'Robert', 'Joe'

The above will replace all instances of Robert with Joe.

If you're dealing with natural language text and need to replace a word, not just part of a string, you have to add a pinch of regular expressions to your gsub as a plain text substitution can lead to disastrous results:

'mislocated cat, vindicating'.gsub('cat', 'dog')=> "mislodoged dog, vindidoging"

Regular expressions have word boundaries, such as \b which matches start or end of a word. Thus,

'mislocated cat, vindicating'.gsub(/\bcat\b/, 'dog')=> "mislocated dog, vindicating"

In Ruby, unlike some other languages like Javascript, word boundaries are UTF-8-compatible, so you can use it for languages with non-Latin or extended Latin alphabets:

'сіль у кисіль, для весіль'.gsub(/\bсіль\b/, 'цукор')=> "цукор у кисіль, для весіль"

You can try using this way :

sentence ["Robert"] = "Roger"

Then the sentence will become :

sentence = "My name is Roger" # Robert is replaced with Roger