JavaScript regular matching Chinese characters, numbers, letters, and underscores_Basic knowledge

Javascript regular detection only contains Chinese characters, numbers, letters, and underscores and cannot start and end with underscores

I encountered a problem with username detection and checked it out. The summary example is as follows:

Copy code The code is as follows:

var reg = /^(?!_)(? !.*?_$)[a-zA-Z0-9_u4e00-u9fa5] $/;
if (!reg.test (actual value string)) {
alert('Please enter 5- 24-bit characters: supports Chinese, English, numbers, "_", Chinese is recommended');
}