Make [:print:] support non-ASCII characters correctly

* src/regex.c (ISPRINT): Call 'printablep' for multibyte characters.
(BIT_PRINT): New bit mask.
(re_wctype_to_bit): Return BIT_PRINT for RECC_PRINT.
* src/character.c (printablep): New function.
* src/character.h (printablep): Add prototype.

* lisp/emacs-lisp/rx.el (rx): Doc fix: document the new behavior
of 'print', 'alnum', and 'alphabetic'.

* doc/lispref/searching.texi (Char Classes): Document the new
behavior of [:print:].

* etc/NEWS: Mention the new behavior of [:print:].

doc/lispref/searching.texi

@@ -569,8 +569,11 @@ This matches any multibyte character (@pxref{Text Representations}).
 @item [:nonascii:]
 This matches any non-@acronym{ASCII} character.
 @item [:print:]
-This matches printing characters---everything except @acronym{ASCII} control
-characters and the delete character.
+This matches printing characters---everything except @acronym{ASCII}
+and non-@acronym{ASCII} control characters (including the delete
+character), surrogates, and codepoints unassigned by Unicode, as
+indicated by the Unicode @samp{general-category} property
+(@pxref{Character Properties}).
 @item [:punct:]
 This matches any punctuation character.  (At present, for multibyte
 characters, it matches anything that has non-word syntax.)

etc/NEWS

@@ -628,6 +628,14 @@ notifications, if Emacs is compiled with file notification support.
 ---
 *** gulp.el
 
++++
+** The character class [:print:] in regular expressions
+no longer matches any multibyte character.  Instead, Emacs now
+consults the Unicode character properties to determine which
+characters are printable.  In particular, surrogates and unassigned
+codepoints are now rejected by this class.  If you want the old
+behavior, use [:multibyte:] instead.
+
 
 * New Modes and Packages in Emacs 25.1
 

lisp/emacs-lisp/rx.el

@@ -969,16 +969,16 @@ CHAR
      space, and DEL.
 
 `printing', `print'
-     matches printing characters--everything except ASCII control chars
-     and DEL.
+     matches printing characters--everything except ASCII and non-ASCII
+     control characters, surrogates, and codepoints unassigned by Unicode.
 
 `alphanumeric', `alnum'
-     matches letters and digits.  (But at present, for multibyte characters,
-     it matches anything that has word syntax.)
+     matches alphabetic characters and digits.  (For multibyte characters,
+     it matches according to Unicode character properties.)
 
 `letter', `alphabetic', `alpha'
-     matches letters.  (But at present, for multibyte characters,
-     it matches anything that has word syntax.)
+     matches alphabetic characters.  (For multibyte characters,
+     it matches according to Unicode character properties.)
 
 `ascii'
      matches ASCII (unibyte) characters.

src/character.c

@@ -1022,6 +1022,22 @@ decimalnump (int c)
   return gen_cat == UNICODE_CATEGORY_Nd;
 }
 
+/* Return 'true' if C is a printable character as defined by its
+   Unicode properties.  */
+bool
+printablep (int c)
+{
+  Lisp_Object category = CHAR_TABLE_REF (Vunicode_category_table, c);
+  if (! INTEGERP (category))
+    return false;
+  EMACS_INT gen_cat = XINT (category);
+
+  /* See UTS #18.  */
+  return (!(gen_cat == UNICODE_CATEGORY_Cc /* control */
+	    || gen_cat == UNICODE_CATEGORY_Cs /* surrogate */
+	    || gen_cat == UNICODE_CATEGORY_Cn)); /* unassigned */
+}
+
 void
 syms_of_character (void)
 {

src/character.h

@@ -662,6 +662,7 @@ extern Lisp_Object string_escape_byte8 (Lisp_Object);
 
 extern bool alphabeticp (int);
 extern bool decimalnump (int);
+extern bool printablep (int);
 
 /* Return a translation table of id number ID.  */
 #define GET_TRANSLATION_TABLE(id) \

src/regex.c

@@ -318,7 +318,7 @@ enum syntaxcode { Swhitespace = 0, Sword = 1, Ssymbol = 2 };
 
 # define ISPRINT(c) (SINGLE_BYTE_CHAR_P (c)				\
 		    ? (c) >= ' ' && !((c) >= 0177 && (c) <= 0237)	\
-		    : 1)
+		     : printablep (c))
 
 # define ISALNUM(c) (IS_REAL_ASCII (c)			\
 		    ? (((c) >= 'a' && (c) <= 'z')	\
@@ -1865,7 +1865,8 @@ struct range_table_work_area
 #define RANGE_TABLE_WORK_ELT(work_area, i) ((work_area).table[i])
 
 /* Bits used to implement the multibyte-part of the various character classes
-   such as [:alnum:] in a charset's range table.  */
+   such as [:alnum:] in a charset's range table.  The code currently assumes
+   that only the low 16 bits are used.  */
 #define BIT_WORD	0x1
 #define BIT_LOWER	0x2
 #define BIT_PUNCT	0x4
@@ -1874,6 +1875,7 @@ struct range_table_work_area
 #define BIT_MULTIBYTE	0x20
 #define BIT_ALPHA	0x40
 #define BIT_ALNUM	0x80
+#define BIT_PRINT	0x100
 
 
 /* Set the bit for character C in a list.  */
@@ -2072,7 +2074,7 @@ re_wctype_to_bit (re_wctype_t cc)
 {
   switch (cc)
     {
-    case RECC_NONASCII: case RECC_PRINT: case RECC_GRAPH:
+    case RECC_NONASCII: case RECC_GRAPH:
     case RECC_MULTIBYTE: return BIT_MULTIBYTE;
     case RECC_ALPHA: return BIT_ALPHA;
     case RECC_ALNUM: return BIT_ALNUM;
@@ -2081,6 +2083,7 @@ re_wctype_to_bit (re_wctype_t cc)
     case RECC_UPPER: return BIT_UPPER;
     case RECC_PUNCT: return BIT_PUNCT;
     case RECC_SPACE: return BIT_SPACE;
+    case RECC_PRINT: return BIT_PRINT;
     case RECC_ASCII: case RECC_DIGIT: case RECC_XDIGIT: case RECC_CNTRL:
     case RECC_BLANK: case RECC_UNIBYTE: case RECC_ERROR: return 0;
     default: