Improve regex printing (#1341)

- Print the raw regex for Flow, just like for Babylon.
- Sort regex flags.

Fixes #1334.

src/printer.js

@@ -878,11 +878,13 @@ function genericPrintNoParens(path, options, print, args) {
     case "ThisExpression":
       return "this";
     case "Super":
-      return "super"; // Babel 6 Literal split
+      return "super";
+    // Babel 6 Literal split
     case "NullLiteral":
-      return "null"; // Babel 6 Literal split
+      return "null";
+    // Babel 6 Literal split
     case "RegExpLiteral":
-      return n.extra.raw;
+      return printRegex(n);
     // Babel 6 Literal split
     case "NumericLiteral":
       return printNumber(n.extra.raw);
@@ -892,6 +894,7 @@ function genericPrintNoParens(path, options, print, args) {
     case "StringLiteral":
     case "Literal":
       if (typeof n.value === "number") return printNumber(n.raw);
+      if (n.regex) return printRegex(n.regex);
       if (typeof n.value !== "string") return "" + n.value;
 
       return nodeStr(n, options); // Babel 6
@@ -3335,6 +3338,11 @@ function makeString(rawContent, enclosingQuote) {
   return enclosingQuote + newContent + enclosingQuote;
 }
 
+function printRegex(node) {
+  const flags = node.flags.split('').sort().join('');
+  return `/${node.pattern}/${flags}`;
+}
+
 function printNumber(rawNumber) {
   return (
     rawNumber

tests/regex/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,8 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`test.js 1`] = `
+/[/]\\/\\u0aBc/mgi;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+/[/]\\/\\u0aBc/gim;
+
+`;

tests/regex/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, null, ["babylon"]);

tests/regex/test.js

@@ -0,0 +1 @@
+/[/]\/\u0aBc/mgi;