How to implement syntax highlighting in your own vanilla JS code editor?

Question

I am working on creating a code editor using JavaScript. It looks something like this -

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<script type="text/javascript" src="https://www.e-learn.cn/qa/static/js/neweditor/ueditor.all.js"></script>
<script type="text/plain" id="editor"  name="content"  style="width:100%;height:200px;"></script>                                 
<script type="text/javascript">
                                 var isueditor=1;
            var editor = UE.getEditor('editor',{
                //这里可以选择自己需要的工具按钮名称,此处仅选择如下五个
                toolbars:[['source','fullscreen',  '|', 'undo', 'redo', '|', 'bold', 'italic', 'underline', 'fontborder', 'strikethrough', 'removeformat', 'formatmatch', 'autotypeset', 'blockquote', 'pasteplain', '|', 'forecolor', 'backcolor', 'insertorderedlist', 'insertunorderedlist', 'selectall', 'cleardoc', '|', 'rowspacingtop', 'rowspacingbottom', 'lineheight', '|', 'customstyle', 'paragraph', 'fontfamily', 'fontsize', '|', 'indent', '|', 'justifyleft', 'justifycenter', 'justifyright', 'justifyjustify', '|', 'link', 'unlink', 'anchor', '|', 'simpleupload', 'insertimage', 'scrawl', 'insertvideo', 'attachment', 'map', 'insertcode', '|', 'horizontal', '|', 'preview', 'searchreplace', 'drafts']],
            
                initialContent:'',
                //关闭字数统计
                wordCount:false,
                zIndex:2,
                //关闭elementPath
                elementPathEnabled:false,
                //默认的编辑区域高度
                initialFrameHeight:250
                //更多其他参数，请参考ueditor.config.js中的配置项
                //更多其他参数，请参考ueditor.config.js中的配置项
            });
                        editor.ready(function() {
            	editor.setDisabled();
            	});
                            $("#editor").find("*").css("max-width","362px");
        </script>              </div>
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                <label for="L_vercode" class="layui-form-label">验证码</label>
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                  <input type="text"  id="code" name="code"   value="" required lay-verify="required" placeholder="图片验证码" autocomplete="off" class="layui-input">
                </div>
                <div class="layui-form-mid">
                  <span style="color: #c00;"><img class="hand" src="https://www.e-learn.cn/qa/user/code.html" onclick="javascript:updatecode();" id="verifycode"><a class="changecode"  href="javascript:updatecode();"> 看不清?</a></span>
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