fixes #19104; peg Incorrect captures [backport:1.6] (#20352)

* fixes #19104; peg Incorrect captures [backport:1.6]

* add tests

Co-authored-by: khchen <khchen@gmail.com>

lib/pure/pegs.nim

@@ -838,10 +838,13 @@ template matchOrParse(mopProc: untyped) =
         var idx = c.ml # reserve a slot for the subpattern
         result = mopProc(s, p.sons[0], start, c)
         if result >= 0:
-          inc(c.ml)
           if idx < MaxSubpatterns:
+            if idx != c.ml:
+              for i in countdown(c.ml, idx):
+                c.matches[i+1] = c.matches[i]
             c.matches[idx] = (start, start+result-1)
           #else: silently ignore the capture
+          inc(c.ml)
       leave(pkCapture, s, p, start, result)
     of pkBackRef:
       enter(pkBackRef, s, p, start)

tests/stdlib/tpegs.nim

@@ -158,6 +158,10 @@ block:
     privateAccess(NonTerminal)
     privateAccess(Captures)
 
+    if "test" =~ peg"s <- {{\ident}}": # bug #19104
+      doAssert matches[0] == "test"
+      doAssert matches[1] == "test", $matches[1]
+
     doAssert escapePeg("abc''def'") == r"'abc'\x27\x27'def'\x27"
     doAssert match("(a b c)", peg"'(' @ ')'")
     doAssert match("W_HI_Le", peg"\y 'while'")