Improve minimum length finder in the presence of back references when there are

multiple groups with the same number.

ChangeLog

@@ -31,11 +31,17 @@ minimum is potentially useful.
 9. Some changes to the way the minimum subject length is handled:
 
    * When PCRE2_NO_START_OPTIMIZE is set, no minimum length is computed; 
-     pcre2test no longer shows a value (of zero).
+     pcre2test omits this item instead of showing a value of zero.
      
    * When no minimum length is set by the normal scan, but a first and/or last 
      code unit is recorded, set the minimum to 1 or 2 as appropriate.
      
+   * When a pattern contains multiple groups with the same number, a back
+     reference cannot know which one to scan for a minimum length. This used to
+     cause the minimum length finder to give up with no result. Now it treats
+     such references as not adding to the minimum length (which it should have 
+     done all along).
+     
 10. A (*MARK) value inside a successful condition was not being returned by the 
 interpretive matcher (it was returned by JIT). This bug has been mended.
 

src/pcre2_study.c

@@ -92,7 +92,6 @@ Returns:   the minimum length
            -1 \C in UTF-8 mode
               or (*ACCEPT)
               or pattern too complicated
-              or back reference to duplicate name/number
            -2 internal error (missing capturing bracket)
            -3 internal error (opcode not listed)
 */
@@ -452,12 +451,12 @@ for (;;)
     that case we must set the minimum length to zero. */
 
     /* Duplicate named pattern back reference. We cannot reliably find a length
-    for this if duplicate numbers are present in the pattern. */
+    for this if duplicate numbers are present in the pattern, so we set the 
+    length to zero here also. */
 
     case OP_DNREF:
     case OP_DNREFI:
-    if (dupcapused) return -1;
-    if ((re->overall_options & PCRE2_MATCH_UNSET_BACKREF) == 0)
+    if (!dupcapused && (re->overall_options & PCRE2_MATCH_UNSET_BACKREF) == 0)
       {
       int count = GET2(cc, 1+IMM2_SIZE);
       PCRE2_UCHAR *slot =
@@ -524,14 +523,13 @@ for (;;)
 
     case OP_REF:
     case OP_REFI:
-    if (dupcapused) return -1;
     recno = GET2(cc, 1);
     if (recno <= backref_cache[0] && backref_cache[recno] >= 0)
       d = backref_cache[recno];
     else
       {
       int i;
-      if ((re->overall_options & PCRE2_MATCH_UNSET_BACKREF) == 0)
+      if (!dupcapused && (re->overall_options & PCRE2_MATCH_UNSET_BACKREF) == 0)
         {
         ce = cs = (PCRE2_UCHAR *)PRIV(find_bracket)(startcode, utf, recno);
         if (cs == NULL) return -2;

testdata/testoutput2

@@ -14607,7 +14607,7 @@ Subject length lower bound = 65535
 Capture group count = 1
 Max back reference = 1
 Starting code units: a b 
-Subject length lower bound = 0
+Subject length lower bound = 1
 
 /(?|(aaa)|(b))(?1)/I
 Capture group count = 1
@@ -14625,7 +14625,7 @@ Max back reference = 1
 Named capture groups:
   a   1
 Starting code units: a b 
-Subject length lower bound = 0
+Subject length lower bound = 1
 
 /(?|(?'a'aaa)|(?'a'b))(?'a'cccc)\k'a'/I,dupnames
 Capture group count = 2
@@ -14636,7 +14636,7 @@ Named capture groups:
 Options: dupnames
 Starting code units: a b 
 Last code unit = 'c'
-Subject length lower bound = 1
+Subject length lower bound = 5
 
 /ab{3cd/
     ab{3cd