compose_ui: Create the compose_ui.replace_syntax function.

`replace_syntax` will replace text inside of a compose textbox. Also,
add tests to the `compose_ui` Node tests for this function.

frontend_tests/node_tests/compose_ui.js

@@ -43,8 +43,12 @@ function make_textbox(s) {
         widget.focused = false;
     };
 
-    widget.val = function () {
-        return widget.s;
+    widget.val = function (new_val) {
+        if (new_val) {
+            widget.s = new_val;
+        } else {
+            return widget.s;
+        }
     };
 
     widget.trigger = function () {
@@ -107,3 +111,12 @@ run_test('smart_insert', () => {
     // like emojis and file links.
 });
 
+run_test('replace_syntax', () => {
+    $('#compose-textarea').val('abcabc');
+
+    compose_ui.replace_syntax('a', 'A');
+    assert.equal($('#compose-textarea').val(), 'Abcabc');
+
+    compose_ui.replace_syntax(/b/g, 'B');
+    assert.equal($('#compose-textarea').val(), 'ABcaBc');
+});

static/js/compose_ui.js

@@ -49,6 +49,19 @@ exports.insert_syntax_and_focus = function (syntax, textarea) {
     exports.smart_insert(textarea, syntax);
 };
 
+exports.replace_syntax = function (old_syntax, new_syntax, textarea) {
+    // Replaces `old_syntax` with `new_syntax` text in the compose box. Due to
+    // the way that JavaScript handles string replacements, if `old_syntax` is
+    // a string it will only replace the first instance. If `old_syntax` is
+    // a RegExp with a global flag, it will replace all instances.
+
+    if (textarea === undefined) {
+        textarea = $('#compose-textarea');
+    }
+
+    textarea.val(textarea.val().replace(old_syntax, new_syntax));
+};
+
 return exports;
 
 }());