Implemented transform-regexp-constructors plugin.

Changes RegExp constructors into literals.

packages/babel-plugin-transform-regexp-constructors/.npmignore

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+src
+__tests__
+node_modules
+*.log

packages/babel-plugin-transform-regexp-constructors/README.md

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+# babel-plugin-transform-regexp-constructors
+
+This changes RegExp constructors into literals if and only if the RegExp
+arguments are string literals.
+
+## Example
+
+**In**
+
+```javascript
+const foo = 'ab+';
+var a = new RegExp(foo+'c', 'i');
+```
+
+**Out**
+
+```javascript
+const foo = 'ab+';
+var a = /ab+c/i;
+```
+
+## Installation
+
+```sh
+$ npm install babel-plugin-transform-regexp-constructors
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "plugins": ["transform-regexp-constructors"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel --plugins transform-regexp-constructors script.js
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  plugins: ["transform-regexp-constructors"]
+});
+```

packages/babel-plugin-transform-regexp-constructors/__tests__/transform-regexp-constructors-test.js

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+jest.autoMockOff();
+
+const babel = require("babel-core");
+const plugin = require("../src/index");
+
+function transform(code) {
+  return babel.transform(code,  {
+    plugins: [plugin],
+  }).code;
+}
+
+describe("transform-regexp-constructors-plugin", () => {
+  it("should transform RegExp constructors with string literals", () => {
+    const source = "var x = new RegExp('ab+c');";
+    const expected = "var x = /ab+c/;";
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should transform RegExp constructors with flags", () => {
+    const source = "var x = new RegExp('ab+c', 'gimuy');";
+    const expected = "var x = /ab+c/gimuy;";
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should transform RegExp escapes", () => {
+    const source = String.raw`var x = new RegExp('\\w+\\s');`;
+    const expected = String.raw`var x = /\w+\s/;`;
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should not transform RegExp constructors with expressions", () => {
+    const source = "var x = new RegExp(foo(), 'g');";
+    const expected = source;
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should transform empty RegExp constructor", () => {
+    const source = "var x = new RegExp();";
+    const expected = "var x = /(?:)/;";
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should transform RegExp constructor with empty string", () => {
+    const source = "var x = new RegExp('');";
+    const expected = "var x = /(?:)/;";
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should resolve expressions and const references", () => {
+    const source = `
+const foo = "ab+";
+const bar = "c\\\\w";
+const flags = "g";
+const ret = new RegExp(foo + bar + "d", flags);`;
+    const expected = `
+const foo = "ab+";
+const bar = "c\\\\w";
+const flags = "g";
+const ret = /ab+c\\wd/g;`;
+    expect(transform(source)).toBe(expected);
+  });
+});

packages/babel-plugin-transform-regexp-constructors/src/index.js

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+"use strict";
+
+module.exports = function({ types: t }) {
+  return {
+    name: "transform-regexp-constructors",
+    visitor: {
+      NewExpression(path) {
+        if (!t.isIdentifier(path.node.callee, {name: "RegExp"})) {
+          return;
+        }
+        const evaluatedArgs = path.get("arguments")
+                                  .map((a) => a.evaluate());
+        if (!evaluatedArgs.every((a) => a.confident === true &&
+            typeof a.value === "string")) {
+          return;
+        }
+        const pattern = (evaluatedArgs.length >= 1 &&
+                          evaluatedArgs[0].value !== "") ?
+                        evaluatedArgs[0].value :
+                        "(?:)";
+        const flags = evaluatedArgs.length >= 2 ?
+                      evaluatedArgs[1].value :
+                      "";
+        path.replaceWith(t.regExpLiteral(pattern, flags));
+      }
+    },
+  };
+};