(perl #131725) ignore the exponent on a decimal float if no digits

Previously the "1e-" in "1e--5" would be treated as "1", but consumed
the "e-".

This wasn't an issue for hex floats.

I considered (and implemented) croaking instead, but this was
inconsistent with the behaviour for hex floats, which only reach this
code if a full hex float has been parsed.

2 files changed, 29 insertions, 2 deletions

diff --git a/t/lib/croak/toke b/t/lib/croak/toke
index 2603224fdd..c477be0336 100644
--- a/t/lib/croak/toke
+++ b/t/lib/croak/toke
@@ -394,3 +394,13 @@ $a = <<~ ;
 
 EXPECT
 Use of bare << to mean <<"" is forbidden at - line 1.
+########
+# NAME incomplete floating point decimal exponent (#131725)
+1e--5
+EXPECT
+Bareword found where operator expected at - line 1, near "1e"
+	(Missing operator before e?)
+Number found where operator expected at - line 1, near "--5"
+	(Missing operator before 5?)
+syntax error at - line 1, near "1e"
+Execution of - aborted due to compilation errors.
diff --git a/toke.c b/toke.c
index 6aa5f2690e..6de7d09ea4 100644
--- a/toke.c
+++ b/toke.c
@@ -11182,9 +11182,11 @@ Perl_scan_num(pTHX_ const char *start, YYSTYPE* lvalp)
               || UNLIKELY(hexfp && isALPHA_FOLD_EQ(*s, 'p')))
             && strchr("+-0123456789_", s[1]))
         {
-            floatit = TRUE;
+            int exp_digits = 0;
+            const char *save_s = s;
+            char * save_d = d;
 
-	    /* regardless of whether user said 3E5 or 3e5, use lower 'e',
+            /* regardless of whether user said 3E5 or 3e5, use lower 'e',
                ditto for p (hexfloats) */
             if ((isALPHA_FOLD_EQ(*s, 'e'))) {
 		/* At least some Mach atof()s don't grok 'E' */
@@ -11216,6 +11218,7 @@ Perl_scan_num(pTHX_ const char *start, YYSTYPE* lvalp)
 	    /* read digits of exponent */
 	    while (isDIGIT(*s) || *s == '_') {
 	        if (isDIGIT(*s)) {
+                    ++exp_digits;
 		    if (d >= e)
 		        Perl_croak(aTHX_ "%s", number_too_long);
 		    *d++ = *s++;
@@ -11227,6 +11230,20 @@ Perl_scan_num(pTHX_ const char *start, YYSTYPE* lvalp)
 		   lastub = s++;
 		}
 	    }
+
+            if (!exp_digits) {
+                /* no exponent digits, the [eEpP] could be for something else,
+                 * though in practice we don't get here for p since that's preparsed
+                 * earlier, and results in only the 0xX being consumed, so behave similarly
+                 * for decimal floats and consume only the D.DD, leaving the [eE] to the
+                 * next token.
+                 */
+                s = save_s;
+                d = save_d;
+            }
+            else {
+                floatit = TRUE;
+            }
 	}