javascript JS_RegExp_function _ && _ negative_lookahead

Posted 2021-05-04

/*
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test
 * Jack(?!Chen)
 * Negative lookahead | only mach Jack OR Jack+(anything not match `Chen`)
 */

let str = 'JackBlack is not my friend.';
let tmpStr = 'JackChen is my friend.';
let withoutG= /Jack(?!Chen)/;
let withG = /Jack(?!Chen)/g;
let withG2 = /Jack(?!Chen)/g;

for(let i = 0; i < 3; i ++) {
	console.log(`Normal WithoutG ${i}th  ` + withoutG.test(str));
}

console.log('===================');

for(let i = 0; i < 3; i ++) {
	console.log(`Normal WithG ${i}th  ` + withG.test(str));
	console.log(`Temp WithG ${i}th  ` + withG.test(tmpStr));
	console.log(`Normal WithG2 ${i}th  ` + withG2.test(str));
}


// Normal  WithoutG 0th  true
// Normal  WithoutG 1th  true
// Normal  WithoutG 2th  true
// ===================
// Normal  WithG 0th  true
// Temp  WithG 0th  false
// Normal  WithG2 0th  true
// Normal  WithG 1th  false
// Temp  WithG 1th  false
// Normal  WithG2 1th  false
// Normal  WithG 2th  false
// Temp  WithG 2th  false
// Normal  WithG2 2th  true