regsub converter: empty match advances by 1 byte to avoid loops

docs/formats.html

@@ -666,6 +666,13 @@ Otherwise <em>precision</em> is the index (counting from 1) of the match to repl
 Without <em>precision</em> (or 0), all matches are replaced.
 </p>
 <p>
+When replacing multiple matches, the next match is searched directly after the currently
+replaced string, so that the <em>subst</em> string itself will never be modified recursively.
+<span class="new">
+However if an empty string is matched, searching advances by 1 character in order to
+avoid matching the same empty string again.</span>
+</p>
+<p>
 In input this converter pre-processes data received from the device before
 following converters read it.
 Converters preceding this one will read unmodified input.

src/RegexpConverter.cc

@@ -197,14 +197,15 @@ static void regsubst(const StreamFormat& fmt, StreamBuffer& buffer, size_t start
             debug("pcre_exec: no match\n");
             break;
         }
-        if (!(fmt.flags & sign_flag) && n < fmt.prec) // without + flag
-        {
-            // do not yet replace this match
-            c += ovector[1];
-            continue;
-        }
-        // replace subexpressions
         l = ovector[1] - ovector[0];
+
+        // no prec: replace all matches
+        // prec with + flag: replace first prec matches
+        // prec without + flag: replace only match number prec
+
+        if ((fmt.flags & sign_flag) || n >= fmt.prec)
+        {
+            // replace subexpressions
             debug("before [%d]= \"%s\"\n", ovector[0], buffer.expand(start+c,ovector[0])());
             debug("match  [%d]= \"%s\"\n", l, buffer.expand(start+c+ovector[0],l)());
             for (r = 1; r < rc; r++)
@@ -243,8 +244,16 @@ static void regsubst(const StreamFormat& fmt, StreamBuffer& buffer, size_t start
                 debug("subs = \"%s\"\n", s.expand()());
             }
             buffer.replace(start+c+ovector[0], l, s);
-        length += s.length() - l;
+            length -= l;
-        c += ovector[0] + s.length();
+            length += s.length();
+            c += s.length();
+        }
+        c += ovector[0];
+        if (l == 0)
+        {
+            debug("pcre_exec: empty match\n");
+            c++; // Empty strings may lead to an endless loop. Match them only once.
+        }
         if (n == fmt.prec) // max match reached
         {
             debug("pcre_exec: max match %d reached\n", n);