mruby-regexp: handle \b inside character class as backspace

Inside `[...]`, `\b` denotes U+0008 (backspace) -- the same as
MRI/Onigmo and PCRE.  parse_escape() was missing the case, so
the backslash was dropped and the bare letter `b` was inserted
into the class.  `[\b]` therefore matched every `b` instead of
backspace.

Add `case 'b': return '\b';` to parse_escape().  The function
is only reached from the character-class body and range
endpoints; the top-level dispatcher emits RE_WBOUND for `\b`
before falling through, so the word-boundary semantics outside
`[...]` are unchanged.

Reported by Sam Ruby in matz/spinel#632; same engine bug
affects both spinel and mruby.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-regexp/src/re_compile.c

@@ -214,6 +214,8 @@ parse_escape(re_compiler *c)
   case 'v': return '\v';
   case 'a': return '\a';
   case 'e': return 0x1b;
+  case 'b': return '\b';  /* backspace; only reachable inside [...] since the
+                             top-level dispatcher emits RE_WBOUND for `\b` */
   default: return ch;  /* literal: \., \\, \/, \(, etc. */
   }
 }

mrbgems/mruby-regexp/test/regexp.rb

@@ -47,6 +47,14 @@ assert("Regexp - character class") do
   assert_equal "abc", md[0]
 end
 
+assert("Regexp - \\b inside character class is backspace") do
+  # Outside [...], \b is the word boundary assertion; inside [...]
+  # it must mean U+0008 (backspace), matching MRI/Onigmo.
+  assert_equal "Ruby", "Ruby".gsub(/[\b]/, "X")
+  assert_equal "aXc", "a\bc".gsub(/[\b]/, "X")
+  assert_equal ["\b", "\t", "\n"], "ABC\b\t\n".scan(/[\b-\n]/)
+end
+
 assert("Regexp - dot") do
   re = Regexp.new("a.c")
   assert_true re.match?("abc")