Require literal '{' in patterns to be escaped

This has been deprecated since v5.16, with a deprecation message
displayed starting in v5.22.

Author: khwilliamson

pod/perldelta.pod

@@ -32,6 +32,17 @@ L</Selected Bug Fixes> section.
 
 [ List each security issue as a =head2 entry ]
 
+=head2 Unescaped literal C<"{"> characters in regular expression
+patterns are no longer permissible
+
+You have to now say something like C<"\{"> or C<"[{]"> to specify to
+match a LEFT CURLY BRACKET.  This will allow future extensions to the
+language.  This restriction is not enforced, nor are there current plans
+to enforce it, if the C<"{"> is the first character in the pattern.
+
+These have been deprecated since v5.16, with a deprecation message
+displayed starting in v5.22.
+
 =head2 Literal control character variable names are no longer permissible
 
 A variable name may no longer contain a literal control character under

pod/perldiag.pod

@@ -6127,18 +6127,21 @@ C<undef *foo>.
 (A) You've accidentally run your script through B<csh> instead of Perl.
 Check the #! line, or manually feed your script into Perl yourself.
 
-=item Unescaped left brace in regex is deprecated, passed through in regex;
+=item Unescaped left brace in regex is illegal in regex;
 marked by S<<-- HERE> in m/%s/
 
-(D deprecated, regexp) You used a literal C<"{"> character in a regular
-expression pattern.  You should change to use C<"\{"> instead, because a
-future version of Perl (tentatively v5.26) will consider this to be a
-syntax error.  If the pattern delimiters are also braces, any matching
+(F) You used a literal C<"{"> character in a regular
+expression pattern.  You should change to use C<"\{"> or C<[{]> instead.
+If the pattern delimiters are also braces, any matching
 right brace (C<"}">) should also be escaped to avoid confusing the parser,
 for example,
 
     qr{abc\{def\}ghi}
 
+This restriction is not enforced if the C<"{"> is the first character in
+the pattern; nor is a warning generated for this case, as there are no
+current plans to forbid it.
+
 =item unexec of %s into %s failed!
 
 (F) The unexec() routine failed for some reason.  See your local FSF

regcomp.c

@@ -13190,14 +13190,13 @@ S_regatom(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth)
 		    } /* End of switch on '\' */
 		    break;
 		case '{':
-		    /* Currently we don't warn when the lbrace is at the start
+		    /* Currently we don't care if the lbrace is at the start
 		     * of a construct.  This catches it in the middle of a
 		     * literal string, or when it's the first thing after
 		     * something like "\b" */
-		    if (! SIZE_ONLY
-			&& (len || (p > RExC_start && isALPHA_A(*(p -1)))))
-		    {
-			ckWARNregdep(p + 1, "Unescaped left brace in regex is deprecated, passed through");
+		    if (len || (p > RExC_start && isALPHA_A(*(p -1)))) {
+                        RExC_parse = p + 1;
+			vFAIL("Unescaped left brace in regex is illegal");
 		    }
 		    /*FALLTHROUGH*/
 		default:    /* A literal character */

t/re/pat_advanced.t

@@ -1159,11 +1159,6 @@ sub run_tests {
     }
 
     {
-        # \, breaks {3,4}
-        no warnings qw{deprecated regexp};
-        ok "xaaay"    !~ /xa{3\,4}y/, '\, in a pattern';
-        ok "xa{3,4}y" =~ /xa{3\,4}y/, '\, in a pattern';
-
         # \c\ followed by _
         ok "x\c_y"    !~ /x\c\_y/,    '\_ in a pattern';
         ok "x\c\_y"   =~ /x\c\_y/,    '\_ in a pattern';

t/re/reg_mesg.t

@@ -268,6 +268,10 @@ my @death =
  '/(?[\ |!])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[\ |!{#}])/',    # [perl #126180]
  '/(?[()-!])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[()-!{#}])/',    # [perl #126204]
  '/(?[!()])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[!(){#}])/',      # [perl #126404]
+ '/\w{/' => 'Unescaped left brace in regex is illegal {#} m/\w{{#}/',
+ '/\q{/' => 'Unescaped left brace in regex is illegal {#} m/\q{{#}/',
+ '/:{4,a}/' => 'Unescaped left brace in regex is illegal {#} m/:{{#}4,a}/',
+ '/xa{3\,4}y/' => 'Unescaped left brace in regex is illegal {#} m/xa{{#}3\,4}y/',
  '/abc/xix' => 'Only one /x regex modifier is allowed',
  '/(?xmsixp:abc)/' => 'Only one /x regex modifier is allowed {#} m/(?xmsixp{#}:abc)/',
  '/(?xmsixp)abc/' => 'Only one /x regex modifier is allowed {#} m/(?xmsixp{#})abc/',
@@ -617,12 +621,6 @@ my @experimental_regex_sets = (
 );
 
 my @deprecated = (
-    '/\w{/' => 'Unescaped left brace in regex is deprecated, passed through {#} m/\w{{#}/',
-    '/\q{/' => [
-                 'Unrecognized escape \q{ passed through {#} m/\q{{#}/',
-                 'Unescaped left brace in regex is deprecated, passed through {#} m/\q{{#}/'
-               ],
-    '/:{4,a}/' => 'Unescaped left brace in regex is deprecated, passed through {#} m/:{{#}4,a}/',
 );
 
 for my $strict ("", "use re 'strict';") {