Raise limit of regexp repetition (Bug#24914)

* src/regex.h (RE_DUP_MAX): Raise limit to 2^16-1.
* etc/NEWS: Announce it.
* doc/lispref/searching.texi (Regexp Backslash): Document it.
* test/src/regex-tests.el (regex-repeat-limit): Test it.

* src/regex.h (reg_errcode_t): Add REG_ESIZEBR code.
* src/regex.c (re_error_msgid): Add corresponding entry.
(GET_INTERVAL_COUNT): Return it instead of the more generic REG_EBADBR
when encountering a repetition greater than RE_DUP_MAX.

* lisp/isearch.el (isearch-search): Don't convert errors starting with
"Invalid" into "incomplete".  Such errors are not incomplete, in the
sense that they cannot be corrected by appending more characters to
the end of the regexp.  The affected error messages are:

- REG_BADPAT "Invalid regular expression"
  - \\(?X:\\) where X is not a legal group number
  - \\_X where X is not < or >

- REG_ECOLLATE "Invalid collation character"
  - There is no code to throw this.

- REG_ECTYPE "Invalid character class name"
  - [[:foo:] where foo is not a valid class name

- REG_ESUBREG "Invalid back reference"
  - \N where N is referenced before matching group N

- REG_BADBR "Invalid content of \\{\\}"
  - \\{N,M\\} where N < 0, M < N, M or N larger than max
  - \\{NX where X is not a digit or backslash
  - \\{N\\X where X is not a }

- REG_ERANGE "Invalid range end"
  - There is no code to throw this.

- REG_BADRPT "Invalid preceding regular expression"
  - We never throw this.  It would usually indicate a "*" with no
    preceding regexp text, but Emacs allows that to match a literal
    "*".

doc/lispref/searching.texi

@@ -642,10 +642,10 @@ is omitted, the minimum is 0; if @var{n} is omitted, there is no
 maximum.  For both forms, @var{m} and @var{n}, if specified, may be no
 larger than
 @ifnottex
-2**15 @minus{} 1
+2**16 @minus{} 1
 @end ifnottex
 @tex
-@math{2^{15}-1}
+@math{2^{16}-1}
 @end tex
 .
 

etc/NEWS

@@ -80,6 +80,13 @@ indirectly, e.g., by checking that functions like
 It blocks line breaking after a one-letter word, also in the case when
 this word is preceded by a non-space, but non-alphanumeric character.
 
++++
+** The limit on repetitions in regexps has been raised to 2^16-1.
+It was previously limited to 2^15-1.  For example, the following
+regular expression was previously invalid, but is now accepted:
+
+   x\{32768\}
+
 
 * Editing Changes in Emacs 27.1
 

lisp/isearch.el

@@ -2858,7 +2858,7 @@ Optional third argument, if t, means if fail just return nil (no error).
      (setq isearch-error (car (cdr lossage)))
      (cond
       ((string-match
-	"\\`Premature \\|\\`Unmatched \\|\\`Invalid "
+	"\\`Premature \\|\\`Unmatched "
 	isearch-error)
        (setq isearch-error "incomplete input"))
       ((and (not isearch-regexp)

src/regex.c

@@ -1194,7 +1194,8 @@ static const char *re_error_msgid[] =
     gettext_noop ("Premature end of regular expression"), /* REG_EEND */
     gettext_noop ("Regular expression too big"), /* REG_ESIZE */
     gettext_noop ("Unmatched ) or \\)"), /* REG_ERPAREN */
-    gettext_noop ("Range striding over charsets") /* REG_ERANGEX  */
+    gettext_noop ("Range striding over charsets"), /* REG_ERANGEX  */
+    gettext_noop ("Invalid content of \\{\\}, repetitions too big") /* REG_ESIZEBR  */
   };
 
 /* Whether to allocate memory during matching.  */
@@ -1915,7 +1916,7 @@ struct range_table_work_area
 	    if (num < 0)						\
 	      num = 0;							\
 	    if (RE_DUP_MAX / 10 - (RE_DUP_MAX % 10 < c - '0') < num)	\
-	      FREE_STACK_RETURN (REG_BADBR);				\
+	      FREE_STACK_RETURN (REG_ESIZEBR);				\
 	    num = num * 10 + c - '0';					\
 	    if (p == pend)						\
 	      FREE_STACK_RETURN (REG_EBRACE);				\

src/regex.h

@@ -270,8 +270,10 @@ extern ptrdiff_t emacs_re_safe_alloca;
 #ifdef RE_DUP_MAX
 # undef RE_DUP_MAX
 #endif
-/* If sizeof(int) == 2, then ((1 << 15) - 1) overflows.  */
-#define RE_DUP_MAX (0x7fff)
+/* Repeat counts are stored in opcodes as 2 byte integers.  This was
+   previously limited to 7fff because the parsing code uses signed
+   ints.  But Emacs only runs on 32 bit platforms anyway.  */
+#define RE_DUP_MAX (0xffff)
 
 
 /* POSIX `cflags' bits (i.e., information for `regcomp').  */
@@ -337,7 +339,8 @@ typedef enum
   REG_EEND,		/* Premature end.  */
   REG_ESIZE,		/* Compiled pattern bigger than 2^16 bytes.  */
   REG_ERPAREN,		/* Unmatched ) or \); not returned from regcomp.  */
-  REG_ERANGEX		/* Range striding over charsets.  */
+  REG_ERANGEX,		/* Range striding over charsets.  */
+  REG_ESIZEBR           /* n or m too big in \{n,m\} */
 } reg_errcode_t;
 
 /* This data structure represents a compiled pattern.  Before calling

test/src/regex-tests.el

@@ -677,4 +677,10 @@ This evaluates the PTESTS test cases from glibc."
 This evaluates the TESTS test cases from glibc."
   (should-not (regex-tests-TESTS)))
 
+(ert-deftest regex-repeat-limit ()
+  "Test the #xFFFF repeat limit."
+  (should (string-match "\\`x\\{65535\\}" (make-string 65535 ?x)))
+  (should-not (string-match "\\`x\\{65535\\}" (make-string 65534 ?x)))
+  (should-error (string-match "\\`x\\{65536\\}" "X") :type 'invalid-regexp))
+
 ;;; regex-tests.el ends here