Returns the integer index of the first match for the given argument, or nil if none found; the search of self is forward, and begins at position offset (in characters).

With string argument substring, returns the index of the first matching substring in self:

'foo'.index('f')         # => 0
'foo'.index('o')         # => 1
'foo'.index('oo')        # => 1
'foo'.index('ooo')       # => nil
'тест'.index('с')        # => 2
'こんにちは'.index('ち')   # => 3

With Regexp argument regexp, returns the index of the first match in self:

'foo'.index(/o./) # => 1
'foo'.index(/.o/) # => 0

With positive integer offset, begins the search at position offset:

'foo'.index('o', 1)        # => 1
'foo'.index('o', 2)        # => 2
'foo'.index('o', 3)        # => nil
'тест'.index('с', 1)       # => 2
'こんにちは'.index('ち', 2)  # => 3

With negative integer offset, selects the search position by counting backward from the end of self:

'foo'.index('o', -1)  # => 2
'foo'.index('o', -2)  # => 1
'foo'.index('o', -3)  # => 1
'foo'.index('o', -4)  # => nil
'foo'.index(/o./, -2) # => 1
'foo'.index(/.o/, -2) # => 1

Related: String#rindex.