diff --git a/build.zig b/build.zig
index ff38efa..1441ddc 100644
--- a/build.zig
+++ b/build.zig
@@ -1,20 +1,18 @@
 const std = @import("std");
 
-// Although this function looks imperative, note that its job is to
-// declaratively construct a build graph that will be executed by an external
-// runner.
 pub fn build(b: *std.Build) void {
-    // Standard target options allows the person running `zig build` to choose
-    // what target to build for. Here we do not override the defaults, which
-    // means any target is allowed, and the default is native. Other options
-    // for restricting supported target set are available.
+    // Standard target options
     const target = b.standardTargetOptions(.{});
-
-    // Standard optimization options allow the person running `zig build` to select
-    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
-    // set a preferred release mode, allowing the user to decide how to optimize.
     const optimize = b.standardOptimizeOption(.{});
 
+    // Create options module for tracing
+    const executionTracing = b.option(bool, "tracing", "enable execution tracing") orelse false;
+    const options = b.addOptions();
+    options.addOption(bool, "tracing", executionTracing);
+
+    // Create the options module that will be shared
+    const options_module = options.createModule();
+
     const exe = b.addExecutable(.{
         .name = "thp",
         .root_source_file = b.path("src/main.zig"),
@@ -22,12 +20,8 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
     });
 
-    // Declare an option to build the program with debug statements
-    const executionTracing = b.option(bool, "tracing", "enable execution tracing") orelse false;
-    const options = b.addOptions();
-    options.addOption(bool, "tracing", executionTracing);
-
-    exe.root_module.addOptions("config", options);
+    // Add options to executable
+    exe.root_module.addImport("config", options_module);
 
     //
     // Error handling module
@@ -37,6 +31,7 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
     });
+    error_module.addImport("config", options_module);
     exe.root_module.addImport("errors", error_module);
 
     //
@@ -47,8 +42,9 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
     });
-    exe.root_module.addImport("lexic", lexic_module);
+    lexic_module.addImport("config", options_module);
     lexic_module.addImport("errors", error_module);
+    exe.root_module.addImport("lexic", lexic_module);
 
     //
     // Syntax module
@@ -58,54 +54,40 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
     });
-    exe.root_module.addImport("syntax", syntax_module);
+    syntax_module.addImport("config", options_module);
     syntax_module.addImport("lexic", lexic_module);
     syntax_module.addImport("errors", error_module);
+    exe.root_module.addImport("syntax", syntax_module);
 
-    // This declares intent for the executable to be installed into the
-    // standard location when the user invokes the "install" step (the default
-    // step when running `zig build`).
+    // Install step
     b.installArtifact(exe);
 
-    // This *creates* a Run step in the build graph, to be executed when another
-    // step is evaluated that depends on it. The next line below will establish
-    // such a dependency.
+    // Run command
     const run_cmd = b.addRunArtifact(exe);
-
-    // By making the run step depend on the install step, it will be run from the
-    // installation directory rather than directly from within the cache directory.
-    // This is not necessary, however, if the application depends on other installed
-    // files, this ensures they will be present and in the expected location.
     run_cmd.step.dependOn(b.getInstallStep());
 
-    // This allows the user to pass arguments to the application in the build
-    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    // Pass arguments if any
     if (b.args) |args| {
         run_cmd.addArgs(args);
     }
 
-    // This creates a build step. It will be visible in the `zig build --help` menu,
-    // and can be selected like this: `zig build run`
-    // This will evaluate the `run` step rather than the default, which is "install".
+    // Run step
     const run_step = b.step("run", "Run the app");
     run_step.dependOn(&run_cmd.step);
 
-    // Creates a step for unit testing. This only builds the test executable
-    // but does not run it.
+    // Unit tests
     const exe_unit_tests = b.addTest(.{
         .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
     });
+    exe_unit_tests.root_module.addImport("config", options_module);
     exe_unit_tests.root_module.addImport("lexic", lexic_module);
     exe_unit_tests.root_module.addImport("syntax", syntax_module);
     exe_unit_tests.root_module.addImport("errors", error_module);
 
     const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests);
 
-    // Similar to creating the run step earlier, this exposes a `test` step to
-    // the `zig build --help` menu, providing a way for the user to request
-    // running the unit tests.
     const test_step = b.step("test", "Run ALL unit tests");
     test_step.dependOn(&run_exe_unit_tests.step);
 
@@ -113,7 +95,6 @@ pub fn build(b: *std.Build) void {
     const files = [_][]const u8{
         "src/01_lexic/root.zig",
         "src/02_syntax/root.zig",
-        "src/02_syntax/root.zig",
         "src/errors/root.zig",
     };
     for (files) |file| {
@@ -122,6 +103,7 @@ pub fn build(b: *std.Build) void {
             .target = target,
             .optimize = optimize,
         });
+        file_unit_test.root_module.addImport("config", options_module);
         file_unit_test.root_module.addImport("lexic", lexic_module);
         file_unit_test.root_module.addImport("syntax", syntax_module);
         file_unit_test.root_module.addImport("errors", error_module);
diff --git a/src/01_lexic/number.zig b/src/01_lexic/number.zig
index d81dfea..16bb257 100644
--- a/src/01_lexic/number.zig
+++ b/src/01_lexic/number.zig
@@ -39,7 +39,7 @@ pub fn lex(
 
     // Attempt to lex an integer.
     // Floating point numbers are lexed through the int lexer
-    return integer(input, cap, start);
+    return integer(input, cap, start, err, alloc);
 }
 
 /// comptime function for lexing hex, octal and binary numbers.
@@ -100,7 +100,13 @@ fn prefixed(
 /// An integer cannot have a leading zero. That is an error to
 /// avoid confussion with PHP literal octals.
 /// Floating point numbers can.
-fn integer(input: []const u8, cap: usize, start: usize) LexError!?LexReturn {
+fn integer(
+    input: []const u8,
+    cap: usize,
+    start: usize,
+    err: *errors.ErrorData,
+    alloc: std.mem.Allocator,
+) LexError!?LexReturn {
     assert(start < cap);
     const first_char = input[start];
     if (!is_decimal_digit(first_char)) {
@@ -119,6 +125,10 @@ fn integer(input: []const u8, cap: usize, start: usize) LexError!?LexReturn {
     if (last_pos >= cap) {
         // leading zero on an integer, throw an error
         if (first_char == '0') {
+            try err.init("Leading zero", start, start + 1, alloc);
+            try err.add_label("This decimal number has a leading zero.", start, last_pos);
+            err.set_help("If you want an octal number use '0o', otherwise remove the leading zero");
+
             return LexError.LeadingZero;
         }
 
@@ -143,6 +153,10 @@ fn integer(input: []const u8, cap: usize, start: usize) LexError!?LexReturn {
         else => {
             // leading zero on an integer, throw an error
             if (first_char == '0') {
+                try err.init("Leading zero", start, start + 1, alloc);
+                try err.add_label("This decimal number has a leading zero.", start, last_pos);
+                err.set_help("If you want an octal number use '0o', otherwise remove the leading zero");
+
                 return LexError.LeadingZero;
             }
 
diff --git a/src/01_lexic/root.zig b/src/01_lexic/root.zig
index 8018b7a..44d504c 100644
--- a/src/01_lexic/root.zig
+++ b/src/01_lexic/root.zig
@@ -45,12 +45,13 @@ pub fn tokenize(
         var current_error: errors.ErrorData = undefined;
         const number_lex = number.lex(input, input_len, actual_next_pos, &current_error, alloc) catch |e| switch (e) {
             // recoverable errors
-            LexError.Incomplete => {
+            LexError.Incomplete, LexError.LeadingZero => {
                 // add to list of errors
                 try err_arrl.append(current_error);
 
                 // ignore everything until whitespace and loop
                 current_pos = ignore_until_whitespace(input, actual_next_pos);
+                assert(current_pos > actual_next_pos);
                 continue;
             },
             // just throw unrecoverable errors
diff --git a/src/main.zig b/src/main.zig
index 06b32de..7ae48ba 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -78,10 +78,10 @@ fn repl() !void {
         // Print errors
         for (error_array.items) |e| {
             var err = e;
-            const err_str = try err.get_error_str(line, "repl", std.heap.page_allocator);
+            const err_str = try err.get_error_str(line, "repl", alloc);
             try stdout.print("\n{s}\n", .{err_str});
             try bw.flush();
-            std.heap.page_allocator.free(err_str);
+            alloc.free(err_str);
         }
 
         // next repl line