The Range Operator (`-')
------------------------

  Regex recognizes "range expressions" inside a list. They represent
those characters that fall between two elements in the current
collating sequence.  You form a range expression by putting a "range
operator" between two characters.(1) `-' represents the range operator.
For example, `a-f' within a list represents all the characters from `a'
through `f' inclusively.

  If the syntax bit `RE_NO_EMPTY_RANGES' is set, then if the range's
ending point collates less than its starting point, the range (and the
regular expression containing it) is invalid.  For example, the regular
expression `[z-a]' would be invalid.  If this bit isn't set, then Regex
considers such a range to be empty.

  Since `-' represents the range operator, if you want to make a `-'
character itself a list item, you must do one of the following:

   * Put the `-' either first or last in the list.

   * Include a range whose starting point collates strictly lower than
     `-' and whose ending point collates equal or higher.  Unless a
     range is the first item in a list, a `-' can't be its starting
     point, but *can* be its ending point.  That is because Regex
     considers `-' to be the range operator unless it is preceded by
     another `-'.  For example, in the ASCII encoding, `)', `*', `+',
     `,', `-', `.', and `/' are contiguous characters in the collating
     sequence.  You might think that `[)-+--/]' has two ranges: `)-+'
     and `--/'.  Rather, it has the ranges `)-+' and `+--', plus the
     character `/', so it matches, e.g., `,', not `.'.

   * Put a range whose starting point is `-' first in the list.

  For example, `[-a-z]' matches a lowercase letter or a hyphen (in
English, in ASCII).

  ---------- Footnotes ----------

  (1)  You can't use a character class for the starting or ending point
of a range, since a character class is not a single character.