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sub! 

        # String.sub!

(from ruby core)
---
    sub!(pattern, replacement)   -> self or nil
    sub!(pattern) {|match| ... } -> self or nil

---

Returns `self` with only the first occurrence (not all occurrences) of
the given `pattern` replaced.

See [Substitution Methods](#class-String-label-Substitution+Methods).

Related: String#sub, String#gsub, String#gsub!.

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