Allow to signal no replacement in gsub with a nil or false replacement.

2 files changed, 19 insertions, 7 deletions

diff --git a/doc/manual.txt b/doc/manual.txt
index f0c379a..ab68394 100644
--- a/doc/manual.txt
+++ b/doc/manual.txt
@@ -230,7 +230,8 @@ below).
   +---------+-----------------------------------+-------------------------+-------------+
   |  patt   |regular expression pattern         |string or userdata       |     n/a     |
   +---------+-----------------------------------+-------------------------+-------------+
-  |  repl   |substitution source                |string, function or table|     n/a     |
+  |  repl   |substitution source                |string, function, table, |     n/a     |
+  |         |                                   |``false`` or ``nil``     |             |
   +---------+-----------------------------------+-------------------------+-------------+
   |   [n]   |maximum number of matches to search| number or function      |   ``nil``   |
   |         |for, or control function, or nil   |                         |             |
@@ -248,9 +249,9 @@ below).
   3. Number of substitutions made.
 
 **Details:**
-  The parameter *repl* can be either a string, a function or a table.
-  On each match made, it is converted into a value *repl_out* that may be used
-  for the replacement.
+  The parameter *repl* can be either a string, a function, a table,
+  ``false`` or ``nil``. On each match made, it is converted into a
+  value *repl_out* that may be used for the replacement.
 
   *repl_out* is generated differently depending on the type of *repl*:
 
@@ -289,6 +290,11 @@ below).
      same rules as for the return value of *repl* call, described in the above
      paragraph.
 
+  4. If *repl* is ``false`` or ``nil``, no replacement is done. Note
+     that, unusually for Lua, if ``repl`` is absent, it is not taken
+     to be ``nil``. This is to prevent programming errors caused by
+     inadvertently missing out *repl*.
+
   Note: Under some circumstances, the value of *repl_out* may be ignored; see
   below_.
 
diff --git a/src/algo.h b/src/algo.h
index 488cad7..2255070 100644
--- a/src/algo.h
+++ b/src/algo.h
@@ -172,8 +172,10 @@ static void checkarg_gsub (lua_State *L, TArgComp *argC, TArgExec *argE) {
   lua_tostring (L, 3);    /* converts number (if any) to string */
   argE->reptype = lua_type (L, 3);
   if (argE->reptype != LUA_TSTRING && argE->reptype != LUA_TTABLE &&
-      argE->reptype != LUA_TFUNCTION) {
-    luaL_typerror (L, 3, "string, table or function");
+      argE->reptype != LUA_TFUNCTION && argE->reptype != LUA_TNIL &&
+      (argE->reptype != LUA_TBOOLEAN ||
+       (argE->reptype == LUA_TBOOLEAN && lua_toboolean (L, 3)))) {
+    luaL_typerror (L, 3, "string, table, function, false or nil");
   }
   argE->funcpos = 3;
   argE->funcpos2 = 4;
@@ -334,7 +336,11 @@ static int algf_gsub (lua_State *L) {
       }
     }
     /*----------------------------------------------------------------*/
-    if (argE.reptype != LUA_TSTRING) {
+    else if (argE.reptype == LUA_TNIL || argE.reptype == LUA_TBOOLEAN) {
+      buffer_addlstring (pBuf, argE.text + from, to - from);
+    }
+    /*----------------------------------------------------------------*/
+    if (argE.reptype == LUA_TTABLE || argE.reptype == LUA_TFUNCTION) {
       if (lua_tostring (L, -1)) {
         buffer_addvalue (pBuf, -1);
         curr_subst = 1;