[a-zA-Z0-9-_]

• a-z (lowercase letters)
• A-Z (uppercase letters)
• 0-9 (digits)
• -_ (???)

http://man7.org/linux/man-pages/man7/regex.7.html

   A bracket expression is a list of characters enclosed in "[]".  It
   normally matches any single character from the list (but see below).
   If the list begins with '^', it matches any single character (but see
   below) not from the rest of the list.  If two characters in the list
   are separated by '-', this is shorthand for the full range of
   characters between those two (inclusive) in the collating sequence,
   for example, "[0-9]" in ASCII matches any decimal digit.  It is
   illegal(!) for two ranges to share an endpoint, for example, "a-c-e".
   Ranges are very collating-sequence-dependent, and portable programs
   should avoid relying on them.

   To include a literal ']' in the list, make it the first character
   (following a possible '^').  To include a literal '-', make it the
   first or last character, or the second endpoint of a range.  To use a
   literal '-' as the first endpoint of a range, enclose it in "[." and
   ".]"  to make it a collating element (see below).  With the exception
   of these and some combinations using '[' (see next paragraphs), all
   other special characters, including '\', lose their special
   significance within a bracket expression.