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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.
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