libstdc++: Add pretty printer for std::regex internals

This helps visualize the NFA states in a std::regex.  It probably isn't
very useful for users, but helps when working on the implementation.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdRegexStatePrinter): New
	printer for std::regex NFA states.

libstdc++-v3/python/libstdcxx/v6/printers.py

@@ -1554,6 +1554,34 @@ class StdErrorCodePrinter:
                 pass
         return '%s = {"%s": %s}' % (self.typename, category, strval)
 
+class StdRegexStatePrinter:
+    "Print a state node in the NFA for a std::regex"
+
+    def __init__ (self, typename, val):
+        self.val = val
+        self.typename = typename
+
+    def to_string (self):
+        opcode = str(self.val['_M_opcode'])
+        if opcode:
+            opcode = opcode[25:]
+        next_id = self.val['_M_next']
+
+        variants = {'repeat':'alt', 'alternative':'alt',
+                    'subexpr_begin':'subexpr', 'subexpr_end':'subexpr',
+                    'line_begin_assertion':None, 'line_end_assertion':None,
+                    'word_boundary':'neg', 'subexpr_lookahead':'neg',
+                    'backref':'backref_index',
+                    'match':None, 'accept':None,
+                    'dummy':None, 'unknown':None
+                   }
+        v = variants[opcode]
+
+        s = "opcode={}, next={}".format(opcode, next_id)
+        if v is not None and self.val['_M_' + v] is not None:
+            s = "{}, {}={}".format(s, v, self.val['_M_' + v])
+        return "{%s}" % (s)
+
 # A "regular expression" printer which conforms to the
 # "SubPrettyPrinter" protocol from gdb.printing.
 class RxPrinter(object):
@@ -2015,6 +2043,10 @@ def build_libstdcxx_dictionary ():
     libstdcxx_printer.add_version('std::tr1::', 'unordered_multiset',
                                   Tr1UnorderedSetPrinter)
 
+    # std::regex components
+    libstdcxx_printer.add_version('std::__detail::', '_State',
+                                  StdRegexStatePrinter)
+
     # These are the C++11 printer registrations for -D_GLIBCXX_DEBUG cases.
     # The tr1 namespace containers do not have any debug equivalents,
     # so do not register printers for them.