Fix a regular expression

This regular expression literal occurs a ruby's warning.

* "character class has duplicated range"

Because "[]" has 2 same characters "|".
I guess real meanings are "(a|b|c)", this regex is able to tranlate "[abc]".

Note
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* #10
* [reverted](https://github.com/kachick/leftright/commit/07a991d1eaff97323cf88dcae09f1029e5b11d44)