Ruby provides Regular Expression replacement via the gsub method of String. For example, if I would like to replace a word I simply need the regex to match the word and the replacement text.

irb(main):006:0> "replacing in regex".gsub /\sin\s/, ' is easy in '
=> "replacing is easy in regex"

The gsub method also lets you use the regex match in the replacement.

irb(main):007:0> "replacing in regex".gsub /\sin\s/, ' is easy\0'
=> "replacing is easy in regex"

If you want part of the regex, but not the whole thing you will need to use a capturing group (parenthesis).

irb(main):008:0> "replacing in regex".gsub /\si(n)\s/, ' is easy i\1 '
=> "replacing is easy in regex"

When using a capturing group that matches multiple times in a single line you can still use \1 to include the match in the result.

irb(main):015:0> "1%, 10%, 100%".gsub /(\d+)%/, '0.0\1'
=> "0.01, 0.010, 0.0100"

Another important detail to note is that I'm using single quotes in my replacement string. Using double quotes neither works with \0 or \1 since "\0" #=> "\000".

irb(main):009:0> "replacing in regex".gsub /\sin\s/, " is easy\0"
=> "replacing is easy\000regex"
irb(main):010:0> "replacing in regex".gsub /\si(n)\s/, " is easy i\1 "
=> "replacing is easy i\001 regex"