Disallow numeric separators at the end of numeric literals

Extend manual grammar inspection to length = 2.

ref #187

scripts/inspect-printable-ascii.js

@@ -1,16 +1,10 @@
 const {parse} = require('../src/sonic-weave-ast');
+const readline = require('readline').createInterface({
+  input: process.stdin,
+  output: process.stdout,
+});
 
-console.log(
-  'Iterating over all printable ASCII character to see which are accepted by the parser.'
-);
-
-const ASCII = ['\t', '\n', '\r'];
-
-for (let i = 32; i < 127; ++i) {
-  ASCII.push(String.fromCharCode(i));
-}
-
-for (const source of ASCII) {
+function logResult(source) {
   let result = '*Empty program*';
   try {
     const ast = parse(source);
@@ -28,3 +22,32 @@ for (const source of ASCII) {
   }
   console.log(JSON.stringify(source) + '\t' + result);
 }
+
+console.log(
+  'Iterating over all printable ASCII character to see which are accepted by the parser.'
+);
+
+const ASCII = ['\t', '\n', '\r'];
+
+for (let i = 32; i < 127; ++i) {
+  ASCII.push(String.fromCharCode(i));
+}
+
+for (const source of ASCII) {
+  logResult(source);
+}
+
+let i = 0;
+
+function nextPage() {
+  for (const b of ASCII) {
+    logResult(ASCII[i] + b);
+  }
+  if (++i < ASCII.length) {
+    readline.question('(next page)', nextPage);
+  } else {
+    readline.close();
+  }
+}
+
+readline.question('Confirm to continue with strings of length 2.', nextPage);

src/__tests__/sonic-weave-ast.spec.ts

@@ -913,3 +913,13 @@ describe('Automatic semicolon insertion', () => {
     expect(ast.body).toHaveLength(2);
   });
 });
+
+describe('Negative tests', () => {
+  it('rejects numbers without digits', () => {
+    expect(() => parse('._')).toThrow();
+  });
+
+  it('rejects trailing underscores in numbers', () => {
+    expect(() => parse('7_')).toThrow();
+  });
+});

src/base.pegjs

@@ -1,8 +1,18 @@
 UnderscoreDigits
-  = num: $([_0-9]*) { return num.replace(/_/g, ''); }
+  = num: $([_0-9]*) {
+  if (num.endsWith('_')) {
+    error('Numeric separators are not allowed at the end of numeric literals.');
+  }
+  return num.replace(/_/g, '');
+}
 
 NonEmptyUnderscoreDigits
-  = num: $([_0-9]+) { return num.replace(/_/g, ''); }
+  = num: $([0-9] [_0-9]*) {
+  if (num.endsWith('_')) {
+    error('Numeric separators are not allowed at the end of numeric literals.');
+  }
+  return num.replace(/_/g, '');
+}
 
 PositiveInteger
   = num:([1-9] UnderscoreDigits) { return BigInt(num.join('')); }