How do I remove the first occurence of a substring in string?

sub will do what you want. gsub is the global version that you're probably already familiar with.

Use sub! to substitute what you are trying to find with ""(nothing), thereby deleting it:

phrase.sub!("foo", "")

The !(bang) at the end makes it permanent. sub is different then gsub in that sub just substitutes the first instance of the string that you are trying to find whereas gsub finds all instances.