In python there's function replace(old, new) which replaces "old" to "new" in some string, is there any function like this or any way to do it in Crystal?
From the Crystal-lang api docs you can use the .sub function:
"hello yellow".sub("ll", "dd") # => "heddo yellow"
Source: https://crystal-lang.org/api/0.23.1/String.html
NOTE: This function only replaces the first occurrence of the search string. There also appears to be a version of the sub function that allows you to pass a regex string which should allow you to grab all occurrences of a particular string.