Regex:

  
NODE                     EXPLANATION
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  ^                        the beginning of the string
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  (                        group and capture to \1 (between 1 and 4
                           times (matching the most amount
                           possible)):
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    1+                       '1' (1 or more times (matching the most
                             amount possible))
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  ){1,4}                   end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
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  $                        before an optional \n, and the end of the
                           string