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index 

        # String.index

(from ruby core)
---
    index(substring, offset = 0) -> integer or nil
    index(regexp, offset = 0) -> integer or nil

---

Returns the Integer index of the first occurrence of the given
`substring`, or `nil` if none found:

    'foo'.index('f') # => 0
    'foo'.index('o') # => 1
    'foo'.index('oo') # => 1
    'foo'.index('ooo') # => nil

Returns the Integer index of the first match for the given Regexp
`regexp`, or `nil` if none found:

    'foo'.index(/f/) # => 0
    'foo'.index(/o/) # => 1
    'foo'.index(/oo/) # => 1
    'foo'.index(/ooo/) # => nil

Integer argument `offset`, if given, specifies the position in the
string to begin the search:

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

If `offset` is negative, counts 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

Related: String#rindex.

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