fix: represent regular expressions as a Regex node

BREAKING CHANGE: Previously, regular expressions were represented as `Identifier` nodes, which hasn't been a problem for decaffeinate because all we wanted to do was pass them through anyway. Now, however, we want to do some processing on them and so need to identify them separately. Not to mention the fact that they are not identifiers!

src/mappers/mapLiteral.ts

@@ -1,10 +1,11 @@
 import SourceType from 'coffee-lex/dist/SourceType';
 import { Literal } from 'decaffeinate-coffeescript/lib/coffee-script/nodes';
 import { inspect } from 'util';
-import { Float, Heregex, Identifier, Int, JavaScript, Node, Quasi, RegexFlags, String, This } from '../nodes';
+import { Float, Heregex, Identifier, Int, JavaScript, Node, Quasi, Regex, RegexFlags, String, This } from '../nodes';
 import isStringAtPosition from '../util/isStringAtPosition';
 import ParseContext from '../util/ParseContext';
 import parseNumber from '../util/parseNumber';
+import parseRegExp from '../util/parseRegExp';
 import parseString from '../util/parseString';
 import mapBase from './mapBase';
 
@@ -50,6 +51,14 @@ export default function mapLiteral(context: ParseContext, node: Literal): Node {
     }
   }
 
+  if (startToken.type === SourceType.REGEXP) {
+    let regExp = parseRegExp(node.value);
+    return new Regex(
+      line, column, start, end, raw, virtual,
+      regExp.pattern, RegexFlags.parse(regExp.flags)
+    );
+  }
+
   if (isStringAtPosition(start, end, context)) {
     return new String(
       line, column, start, end, raw, virtual,

src/nodes.ts

@@ -334,6 +334,7 @@ export class Null extends Node {
     super('Null', line, column, start, end, raw, virtual);
   }
 }
+
 export class Undefined extends Node {
   constructor(
     line: number,
@@ -347,6 +348,26 @@ export class Undefined extends Node {
   }
 }
 
+export class Regex extends Node {
+  readonly pattern: string;
+  readonly flags: RegexFlags;
+
+  constructor(
+    line: number,
+    column: number,
+    start: number,
+    end: number,
+    raw: string,
+    virtual: boolean,
+    pattern: string,
+    flags: RegexFlags
+  ) {
+    super('Regex', line, column, start, end, raw, virtual);
+    this.pattern = pattern;
+    this.flags = flags;
+  }
+}
+
 export class Return extends Node {
   readonly argument: Node | null;
 

src/util/parseRegExp.ts

@@ -0,0 +1,8 @@
+import { parse } from 'babylon';
+
+/**
+ * Parses JavaScript source representing a regular expression.
+ */
+export default function parseRegExp(string: string): { pattern: string, flags: string } {
+  return parse(`(${string})`).program.body[0].expression;
+}

test/examples/regexp/input.coffee

@@ -0,0 +1 @@
+/a/

test/examples/regexp/output.json

@@ -0,0 +1,43 @@
+{
+  "type": "Program",
+  "line": 1,
+  "column": 1,
+  "range": [
+    0,
+    4
+  ],
+  "raw": "/a/\n",
+  "body": {
+    "type": "Block",
+    "line": 1,
+    "column": 1,
+    "range": [
+      0,
+      3
+    ],
+    "statements": [
+      {
+        "type": "Regex",
+        "line": 1,
+        "column": 1,
+        "range": [
+          0,
+          3
+        ],
+        "raw": "/a/",
+        "pattern": "a",
+        "flags": {
+          "global": false,
+          "ignoreCase": false,
+          "multiline": false,
+          "sticky": false,
+          "g": false,
+          "i": false,
+          "m": false,
+          "y": false
+        }
+      }
+    ],
+    "raw": "/a/"
+  }
+}