From 05e7c2aff2e459afff7a6a077668891aafe7a834 Mon Sep 17 00:00:00 2001
From: Tomas Babej <tomas@tbabej.com>
Date: Sun, 24 Jan 2021 01:29:34 -0500
Subject: [PATCH] Lexer: Do not allow leading zero for two and more digit
 integers

---
 src/Lexer.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/Lexer.cpp b/src/Lexer.cpp
index 9da16cd2c..b42d5f3ef 100644
--- a/src/Lexer.cpp
+++ b/src/Lexer.cpp
@@ -687,17 +687,25 @@ bool Lexer::isNumber (std::string& token, Lexer::Type& type)
 
 ////////////////////////////////////////////////////////////////////////////////
 // Lexer::Type::number
-//   \d+
+//   0
+//   [1-9]\d*
+//   Integers do not start with a leading 0, unless they are zero.
 bool Lexer::isInteger (std::string& token, Lexer::Type& type)
 {
   std::size_t marker = _cursor;
 
+  bool leading_zero = (_text[marker] == '0');
+
   if (unicodeLatinDigit (_text[marker]))
   {
     ++marker;
     while (unicodeLatinDigit (_text[marker]))
       utf8_next_char (_text, marker);
 
+    // Leading zero is only allowed in the case of number 0
+    if (leading_zero and marker - _cursor > 1)
+      return false;
+
     token = _text.substr (_cursor, marker - _cursor);
     type = Lexer::Type::number;
     _cursor = marker;