String#delete returns a copy of the String object it is called upon, and deletes characters at the intersection of its arguments. The rules are the same as the String#count method, therefore it is important to understand this method first.
The count Method
The String#count method is called passing in one or more argument and returns an Integer. When two arguments are passed in, the return value is the count of the characters ate the intersection of the passed arguments.
The intersection is a set containing all the elements common to the first and second set. Here, the sets are the passed arguments. It can be a String object ('e') or a sequence ('a-d', which translates to a, b, c and d). Sequences shall not be confused with multiple characters: a-d is different than ad ; the later translate to a and d.
In other words, if a characters appears in the first argument (the first set) and in the second argument (the second set), it is at the intersection of both arguments/sets and, therefore, will be counted.
Examples
Given a String object panda, let’s find and count the characters at the intersection of an and a.
For the sake of simplicity, we will number each characters as follow:
panda
12345'panda'.count('an') # => 3
# Counts characters 2, 3 and 5 (set A)
'panda'.count('a') # => 2
# Counts characters 2 and 5 (set B)
'panda'.count('an', 'a') # => 2
# Counts characters at the intersection of sets A and B (2 and 5)
Given our new knowledge, let’s check how the delete method works.
The delete Method
The delete method works like the count method. It deletes the characters at the intersection of both parameters:
'panda'.delete('an') # => pd
# Deletes characters 2, 3 and 5 (set A)
'panda'.delete('a') # => pnd
# Deletes characters 2 and 5 (set B)
'panda'.delete('an', 'a') # => pnd
# Deletes characters at the intersection of sets A and B (2 and 5)