regcomp: Improve error message for (?-d:...)

As agreed, this improvement is going into 5.14.  A customized
message is output, instead of a generic one.

3 files changed, 20 insertions, 8 deletions

diff --git a/pod/perldiag.pod b/pod/perldiag.pod
index e9c5543871..88c55a8e68 100644
--- a/pod/perldiag.pod
+++ b/pod/perldiag.pod
@@ -4021,6 +4021,13 @@ expression compiler gave it.
 (F syntax, regexp) The regular expression pattern had too many occurrences
 of the specified modifier.  Remove the extraneous ones.
 
+=item Regexp modifier "%c" may not appear after the "-"
+
+(F regexp) Turning off the given modifier has the side effect of turning
+on another one.  Perl currently doesn't allow this.  Reword the regular
+expression to use the modifier you want to turn on (and place it before
+the minus), instead of the one you want to turn off.
+
 =item Regexp modifiers "/%c" and "/%c" are mutually exclusive
 
 (F syntax, regexp) The regular expression pattern had more than one of these
@@ -4168,8 +4175,7 @@ where the problem was discovered. See L<perlre>.
 <-- HERE shows in the regular expression about where the problem was
 discovered.  This happens when using the C<(?^...)> construct to tell
 Perl to use the default regular expression modifiers, and you
-redundantly specify a default modifier; or having a modifier that can't
-be turned off (such as C<"p"> or C<"l">) after a minus.  For other
+redundantly specify a default modifier.  For other
 causes, see L<perlre>.
 
 =item Sequence \%s... not terminated in regex; marked by <-- HERE in m/%s/
diff --git a/regcomp.c b/regcomp.c
index e37bc46c2a..085884174a 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -7086,7 +7086,7 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I32 paren, I32 *flagp,U32 depth)
 			    goto excess_modifier;
 			}
 			else if (flagsp == &negflags) {
-                            goto fail_modifiers;
+                            goto neg_modifier;
                         }
 			cs = REGEX_LOCALE_CHARSET;
                         has_charset_modifier = LOCALE_PAT_MOD;
@@ -7097,14 +7097,14 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I32 paren, I32 *flagp,U32 depth)
 			    goto excess_modifier;
 			}
 			else if (flagsp == &negflags) {
-                            goto fail_modifiers;
+                            goto neg_modifier;
                         }
 			cs = REGEX_UNICODE_CHARSET;
                         has_charset_modifier = UNICODE_PAT_MOD;
                         break;
                     case ASCII_RESTRICT_PAT_MOD:
                         if (flagsp == &negflags) {
-                            goto fail_modifiers;
+                            goto neg_modifier;
                         }
                         if (has_charset_modifier) {
                             if (cs != REGEX_ASCII_RESTRICTED_CHARSET) {
@@ -7119,11 +7119,12 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I32 paren, I32 *flagp,U32 depth)
                         has_charset_modifier = ASCII_RESTRICT_PAT_MOD;
                         break;
                     case DEPENDS_PAT_MOD:
-                        if (has_use_defaults
-                            || flagsp == &negflags)
-                        {
+                        if (has_use_defaults) {
                             goto fail_modifiers;
 			}
+			else if (flagsp == &negflags) {
+                            goto neg_modifier;
+			}
 			else if (has_charset_modifier) {
 			    goto excess_modifier;
                         }
@@ -7149,6 +7150,10 @@ S_reg(pTHX_ RExC_state_t *pRExC_state, I32 paren, I32 *flagp,U32 depth)
 			    vFAIL3("Regexp modifiers \"%c\" and \"%c\" are mutually exclusive", has_charset_modifier, *(RExC_parse - 1));
 			}
 			/*NOTREACHED*/
+		    neg_modifier:
+			RExC_parse++;
+			vFAIL2("Regexp modifier \"%c\" may not appear after the \"-\"", *(RExC_parse - 1));
+			/*NOTREACHED*/
                     case ONCE_PAT_MOD: /* 'o' */
                     case GLOBAL_PAT_MOD: /* 'g' */
 			if (SIZE_ONLY && ckWARN(WARN_REGEXP)) {
diff --git a/t/re/reg_mesg.t b/t/re/reg_mesg.t
index 976798c132..d6b343b1c3 100644
--- a/t/re/reg_mesg.t
+++ b/t/re/reg_mesg.t
@@ -69,6 +69,7 @@ my @death =
 '/(?da:foo)/' => 'Regexp modifiers "d" and "a" are mutually exclusive in regex; marked by {#} in m/(?da{#}:foo)/',
 '/(?lil:foo)/' => 'Regexp modifier "l" may not appear twice in regex; marked by {#} in m/(?lil{#}:foo)/',
 '/(?aaia:foo)/' => 'Regexp modifier "a" may appear a maximum of twice in regex; marked by {#} in m/(?aaia{#}:foo)/',
+'/(?i-l:foo)/' => 'Regexp modifier "l" may not appear after the "-" in regex; marked by {#} in m/(?i-l{#}:foo)/',
 
  '/((x)/' => 'Unmatched ( in regex; marked by {#} in m/({#}(x)/',