diff --git a/assets/parser.c.erb b/assets/parser.c.erb
index 4736949..0d16a9f 100644
--- a/assets/parser.c.erb
+++ b/assets/parser.c.erb
@@ -981,7 +981,7 @@ static size_t parse_from(<%= @grammar.prefix %>context_t * context, size_t start
     {
         if (token == INVALID_TOKEN_ID)
         {
-            size_t lexer_result = <%= @grammar.prefix %>lex(context, &token_info);
+            size_t lexer_result = <%= lex_fn %>(context, &token_info);
             if (lexer_result != P_SUCCESS)
             {
                 result = lexer_result;
diff --git a/assets/parser.d.erb b/assets/parser.d.erb
index 64b3dd7..99217ad 100644
--- a/assets/parser.d.erb
+++ b/assets/parser.d.erb
@@ -86,6 +86,14 @@ public union <%= @grammar.prefix %>value_t
     <%= typestring %> v_<%= name %>;
 <%   end %>
 }
+
+/** Parser value constructor(s). */
+<%   @grammar.ptypes.each do |name, typestring| %>
+public <%= @grammar.prefix %>value_t <%= @grammar.prefix %>value<%= name == "default" ? "" : "_#{name}" %>(T)(T v)
+{
+    return <%= @grammar.prefix %>value_t(v_<%= name %>: v);
+}
+<%   end %>
 <% end %>
 
 <% if @grammar.tree %>
@@ -1037,7 +1045,7 @@ private size_t parse_from(<%= @grammar.prefix %>context_t * context, size_t star
     {
         if (token == INVALID_TOKEN_ID)
         {
-            size_t lexer_result = <%= @grammar.prefix %>lex(context, &token_info);
+            size_t lexer_result = <%= lex_fn %>(context, &token_info);
             if (lexer_result != P_SUCCESS)
             {
                 return lexer_result;
diff --git a/assets/parser.h.erb b/assets/parser.h.erb
index 606d0be..32f3139 100644
--- a/assets/parser.h.erb
+++ b/assets/parser.h.erb
@@ -69,6 +69,14 @@ typedef union
     <%= typestring %> v_<%= name %>;
 <%   end %>
 } <%= @grammar.prefix %>value_t;
+
+/** Parser value constructor(s). */
+<%   @grammar.ptypes.each do |name, typestring| %>
+static inline <%= @grammar.prefix %>value_t <%= @grammar.prefix %>value<%= name == "default" ? "" : "_#{name}" %>(<%= typestring %> v)
+{
+    return (<%= @grammar.prefix %>value_t){.v_<%= name %> = v};
+}
+<%   end %>
 <% end %>
 
 <% if @grammar.tree %>
diff --git a/doc/user_guide.md b/doc/user_guide.md
index 3608941..fe4bf3a 100644
--- a/doc/user_guide.md
+++ b/doc/user_guide.md
@@ -316,6 +316,61 @@ free_token_node <<
 The `free_token_node` statement user code block is not emitted for D language
 since D has a garbage collector.
 
+##> `lex_fn` statement - specifying a custom lexer function
+
+Propane generates both a lexer and a parser.
+By default, the parser uses the generated `p_lex()` function directly to
+return information for the next lexed token from the input stream.
+
+However, the user can specify a custom lex function.
+This function may or may not use the Propane generated `p_lex()` function under
+the hood.
+For example, a token sequence could be injected or repeated from a previously
+saved macro definition.
+
+Example (C/C++):
+
+```
+<<
+static size_t mylexfn(p_context_t * context, p_token_info_t * out_token_info)
+{
+    static size_t count;
+    size_t result = P_SUCCESS;
+    if (count > 0)
+    {
+        out_token_info->token = TOKEN_a;
+        out_token_info->pvalue = p_value(count);
+        count--;
+    }
+    else
+    {
+        result = p_lex(context, out_token_info);
+        if (out_token_info->token == TOKEN_c)
+        {
+            count = 3;
+        }
+    }
+    return result;
+}
+>>
+
+lex_fn mylexfn;
+```
+
+The `lex_fn` statement takes one argument specifying the name of the custom
+lexer function.
+The user must supply a value for the `token` field of the `p_token_info_t`
+output structure so that the parser knows what token was lexed.
+
+Additionally, if the parser user code makes use of the token's pvalue, then
+the lexer function must supply a value for the `pvalue` field of the
+`p_token_info_t` structure.
+The `p_value()` generated API function could be useful for specifying
+parser values to associate with the lexed token when tree generation is not
+enabled.
+When tree generation is enabled, the `pvalue` field can be set to an instance
+of whatever type the user has defined as the `ptype` type.
+
 ##> `module` statement - specifying the generated parser module name
 
 The `module` statement can be used to specify the module name for a generated
@@ -1007,6 +1062,17 @@ For C targets, the `p_position_t` structure can be checked for validity by
 calling `p_position_valid(pos)` where `pos` is a `p_position_t` structure
 instance.
 
+### `p_value_t`
+
+If tree generation mode is enabled, the `p_value_t` type is defined to be the
+type given to the `ptype` statement in the grammar file.
+
+If tree generation mode is not enabled, there could be more than one `ptype`
+given, so the `p_value_t` type is a union of all possible `ptype` types.
+In this case, the API functions `p_value()` and `p_value_XXX()` for each given
+`ptype` name `XXX` are generated to return `p_value_t` instances holding the
+corresponding `ptype`.
+
 ### `p_token_info_t`
 
 The `p_token_info_t` structure contains the following fields:
@@ -1336,6 +1402,20 @@ p_tree_delete_Statement(statement_tree);
 In this case, Propane will free a `Statement` tree structure returned by the
 `p_parse_Statement(context)` function.
 
+### `p_value`
+
+When tree generation mode is not active, the `p_value_t` union can hold one of
+several different possible value types.
+
+The `p_value(v)` function returns an instance of the `p_value_t` with the
+default member set to the value of `v`.
+
+A `p_value_XXX(v)` function is set for each user-defined `ptype` name with
+the user-given name in place of the `XXX`.
+
+These functions are useful for custom lexer functions which need to return a
+parser value corresponding to a lexed token.
+
 ##> Data
 
 ### `p_token_names`
diff --git a/lib/propane/generator.rb b/lib/propane/generator.rb
index 087df31..c20483c 100644
--- a/lib/propane/generator.rb
+++ b/lib/propane/generator.rb
@@ -354,6 +354,14 @@ class Propane
       code
     end
 
+    # Get the lex function to use.
+    #
+    # @return [String]
+    #   Lex function to use.
+    def lex_fn
+      @grammar.lex_fn || "#{@grammar.prefix}lex"
+    end
+
     # Get the parser value type for the start rule.
     #
     # @return [Array<String>]
diff --git a/lib/propane/grammar.rb b/lib/propane/grammar.rb
index 095a418..f8e6569 100644
--- a/lib/propane/grammar.rb
+++ b/lib/propane/grammar.rb
@@ -6,6 +6,7 @@ class Propane
     IDENTIFIER_REGEX = /(?:[a-zA-Z]|_[a-zA-Z0-9])[a-zA-Z_0-9]*/
 
     attr_reader :context_user_fields
+    attr_reader :lex_fn
     attr_reader :tree
     attr_reader :tree_prefix
     attr_reader :tree_suffix
@@ -69,6 +70,7 @@ class Propane
       elsif parse_comment_line!
       elsif @modeline.nil? && parse_mode_label!
       elsif parse_context_user_fields_statement!
+      elsif parse_lex_fn!
       elsif parse_tree_statement!
       elsif parse_tree_prefix_statement!
       elsif parse_tree_suffix_statement!
@@ -117,6 +119,12 @@ class Propane
       end
     end
 
+    def parse_lex_fn!
+      if md = consume!(/lex_fn\b\s*(\w+)\s*;/)
+        @lex_fn = md[1]
+      end
+    end
+
     def parse_tree_statement!
       if consume!(/tree\s*;/)
         @tree = true
diff --git a/spec/propane_spec.rb b/spec/propane_spec.rb
index bdf4b2f..384d8d6 100644
--- a/spec/propane_spec.rb
+++ b/spec/propane_spec.rb
@@ -1699,6 +1699,88 @@ EOF
         results = run_test(language: language)
         expect(results.status).to eq 0
       end
+
+      it "allows a custom lex function" do
+        if language == "d"
+          write_grammar <<EOF
+<<
+private size_t mylexfn(p_context_t * context, p_token_info_t * out_token_info)
+{
+    static size_t count;
+    size_t result = P_SUCCESS;
+    if (count > 0)
+    {
+        out_token_info.token = TOKEN_a;
+        out_token_info.pvalue = p_value(count);
+        count--;
+    }
+    else
+    {
+        result = p_lex(context, out_token_info);
+        if (out_token_info.token == TOKEN_c)
+        {
+            count = 3;
+        }
+    }
+    return result;
+}
+>>
+
+ptype size_t;
+lex_fn mylexfn;
+
+token a << $$ = 7; >>
+token b << $$ = 8; >>
+token c << $$ = 9; >>
+Start -> << $$ = 0; >>
+Start -> Start ID << $$ = ($1 << 4) | $2; >>
+ID -> a << $$ = $1; >>
+ID -> b << $$ = $1; >>
+ID -> c << $$ = $1; >>
+EOF
+        else
+          write_grammar <<EOF
+<<
+static size_t mylexfn(p_context_t * context, p_token_info_t * out_token_info)
+{
+    static size_t count;
+    size_t result = P_SUCCESS;
+    if (count > 0)
+    {
+        out_token_info->token = TOKEN_a;
+        out_token_info->pvalue = p_value(count);
+        count--;
+    }
+    else
+    {
+        result = p_lex(context, out_token_info);
+        if (out_token_info->token == TOKEN_c)
+        {
+            count = 3;
+        }
+    }
+    return result;
+}
+>>
+
+ptype size_t;
+lex_fn mylexfn;
+
+token a << $$ = 7; >>
+token b << $$ = 8; >>
+token c << $$ = 9; >>
+Start -> << $$ = 0; >>
+Start -> Start ID << $$ = ($1 << 4) | $2; >>
+ID -> a << $$ = $1; >>
+ID -> b << $$ = $1; >>
+ID -> c << $$ = $1; >>
+EOF
+        end
+        run_propane(language: language)
+        compile("spec/test_custom_lex_fn.#{language}", language: language)
+        results = run_test(language: language)
+        expect(results.status).to eq 0
+      end
     end
   end
 end
diff --git a/spec/test_custom_lex_fn.c b/spec/test_custom_lex_fn.c
new file mode 100644
index 0000000..7b4439d
--- /dev/null
+++ b/spec/test_custom_lex_fn.c
@@ -0,0 +1,15 @@
+#include "testparser.h"
+#include "testutils.h"
+#include <string.h>
+
+int main()
+{
+    char const * input = "cbacba";
+    p_context_t * context = p_context_new((uint8_t const *)input, strlen(input));
+    assert_eq(P_SUCCESS, p_parse(context));
+    size_t result = p_result(context);
+    assert_eq(0x932187932187, result);
+    p_context_delete(context);
+
+    return 0;
+}
diff --git a/spec/test_custom_lex_fn.d b/spec/test_custom_lex_fn.d
new file mode 100644
index 0000000..76db2e3
--- /dev/null
+++ b/spec/test_custom_lex_fn.d
@@ -0,0 +1,18 @@
+import testparser;
+import std.stdio;
+import testutils;
+
+int main()
+{
+    return 0;
+}
+
+unittest
+{
+    string input = "cbacba";
+    p_context_t * context = p_context_new(input);
+    assert_eq(P_SUCCESS, p_parse(context));
+    size_t result = p_result(context);
+    assert_eq(0x932187932187, result);
+    p_context_delete(context);
+}