the meaning of [ ] in a regular expression [- ] ?

Yes, Ruby's regular expression [-] matches one character and is equivalent to the regular expression -.

Although not explicitly written in the Ruby 2.5 document (this or this), hyphens in [] are treated as expressing a range only when [a-z].Nonmetacharacters without backslash escape are treated as nonmetacharacters, such as when at the beginning.

For example, the regular expression [-abc] matches -, a, b, and c (Example: Rubular/"https://wandbox.org/permlink/jrOb0pT4Kk6UszvV"rel="norever/norebox>.

Reference Blog: Regular Expressions: Symbols that do not need to be escaped within character class [] --Tech Racho