Fix incorrect syntax highlighting for variables starting with "function" (#992)

* closes #982

* fixed pathing bug in testing

Author: gowerc

src/test/suite/syntax.test.ts

@@ -0,0 +1,73 @@
+'use strict';
+
+import { readFileSync } from 'fs';
+import { join } from 'path';
+import * as assert from 'assert';
+
+interface syntaxFile {
+    'repository': {
+        'function-declarations': {
+            'patterns': [
+                {
+                    'match':string
+                }
+            ]
+        }
+    }
+}
+
+
+/**
+ * Converts a POSIX regular expression string into a format
+ * that JavaScript can use. This is because vscode parses 
+ * syntax files using POSIX but JavaScript can't support it.
+ * @param {string} s - A string to be used as a regular expression
+ */
+const regex_from_posix = function (s: string): string {
+    const mappings = [
+        ['[:alnum:]', 'a-zA-Z0-9'],
+        ['[:alpha:]', 'a-zA-Z']
+    ];
+    let s2 = s;
+    mappings.forEach((el) => {
+        s2 = s2.replace(el[0], el[1]);
+    });
+    return s2;
+};
+
+
+const extension_root: string = join(__dirname, '..', '..', '..');
+
+const r_syntax_file: string = join(extension_root, 'syntax', 'r.json');
+console.log(r_syntax_file);
+const rsyntax_raw = readFileSync(r_syntax_file) as unknown;
+const rsyntax: syntaxFile = JSON.parse(rsyntax_raw as string) as syntaxFile;
+
+const function_pattern: string = rsyntax.repository['function-declarations'].patterns[0].match;
+const function_pattern_fixed: string = regex_from_posix(function_pattern);
+
+
+suite('Syntax Highlighting', () => {
+
+    test('function-declarations - basic match', () => {
+        const re = new RegExp(function_pattern_fixed);
+        const line = 'x <- function(x) {';
+        const match = re.exec(line);
+        assert.strictEqual(match[3], 'function');
+    });
+    
+    test('function-declarations - extra spacing', () => {
+        const re = new RegExp(function_pattern_fixed);
+        const line = 'x <- function  (x) {';
+        const match = re.exec(line);
+        assert.strictEqual(match[3], 'function');
+    });
+    
+    test('function-declarations - false function', () => {
+        const re = new RegExp(function_pattern_fixed);
+        const line = 'x <- functions';
+        const match = re.exec(line);
+        assert.strictEqual(match, null);
+    });
+
+});

syntax/r.json

@@ -440,7 +440,7 @@
         "function-declarations": {
             "patterns": [
                 {
-                    "match": "((?:`[^`\\\\]*(?:\\\\.[^`\\\\]*)*`)|(?:[[:alpha:].][[:alnum:]._]*))\\s*(<?<-|=(?!=))\\s*(function)",
+                    "match": "((?:`[^`\\\\]*(?:\\\\.[^`\\\\]*)*`)|(?:[[:alpha:].][[:alnum:]._]*))\\s*(<?<-|=(?!=))\\s*(function)(?!\\w)",
                     "captures": {
                         "1": {
                             "name": "entity.name.function.r"