本篇内容主要讲解“JavaScript正则表达式中g标志实例分析”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“JavaScript正则表达式中g标志实例分析”吧!
有一天在思否社区看到有个问题，大致描述如下
const list = ['a', 'b', '-', 'c', 'd'];const reg = /[a-z]/g;const letters = list.filter(i => reg.test(i));// letters === ['a', 'c'];// 如果正则不使用`g`标志可以得到所有的字母// 为什么加入`g`之后就不可以了对问题而言，遍历中的i就是一个字符，不需要用到g。
但是就我对正则的理解（过于浅薄）感觉上有没有g（只是全局搜索，不会匹配到就停下来）应该不影响，激发了我的好奇心。
上面题的建议写法如下
const reg = /[a-z]/g;reg.test('a'); // => truereg.test('a'); // => falsereg.test('a'); // => truereg.test('a'); // => falsereg.test('a'); // => true解密过程
首先可以确定的表现一定是g导致的
搜索引擎
打开 MDN 仔细查看g标志的作用，得到结论和我的理解无二。
我猜想应该就是g可能启用了某种缓存，又因为reg相对过滤器是全局变量，我将代码改为：
const list = ['a', 'b', '-', 'c', 'd'];const letters = list.filter(i => /[a-z]/g.test(i));// letters === ['a', 'b', 'c', 'd'];将正则声明到每一次遍历，得到结论就是正确的，验证了我的猜想。也得到了，缓存就是正则中的某个地方
下面我找到对应的源码来查看问题的原因
源码层面
由于最近在看 Rust，所以使用 Rust 编写的源码查看
打开项目后，点击.进入 vscode 模式，command+p 搜索 regexp 关键词
进入test.rs文件，command+f 搜索/g可以找到在 90 行有个last_index()的测试
#[test]fn last_index() { let mut context = Context::default(); let init = r#" var regex = /[0-9]+(\.[0-9]+)?/g; "#; // forward 的作用：更改 context，并返回结果的字符串。 eprintln!("{}", forward(&mut context, init)); assert_eq!(forward(&mut context, "regex.lastIndex"), "0"); assert_eq!(forward(&mut context, "regex.test('1.0foo')"), "true"); assert_eq!(forward(&mut context, "regex.lastIndex"), "3"); assert_eq!(forward(&mut context, "regex.test('1.0foo')"), "false"); assert_eq!(forward(&mut context, "regex.lastIndex"), "0");}看到了有lastIndex关键字，这里再已经大致猜到问题的原因了，g 标志存在匹配后的最后一个下标，导致出现问题。
我们将视线移入到mod.rs文件中，搜索test
在 631 行看到了fn test()方法
pub(crate) fn test( this: &JsValue, args: &[JsValue], context: &mut Context,) -> JsResult<JsValue> { // 1. Let R be the this value. // 2. If Type(R) is not Object, throw a TypeError exception. let this = this.as_object().ok_or_else(|| { context .construct_type_error("RegExp.prototype.test method called on incompatible value") })?; // 3. Let string be ? ToString(S). let arg_str = args .get(0) .cloned() .unwrap_or_default() .to_string(context)?; // 4. Let match be ? RegExpExec(R, string). let m = Self::abstract_exec(this, arg_str, context)?; // 5. If match is not null, return true; else return false. if m.is_some() { Ok(JsValue::new(true)) } else { Ok(JsValue::new(false)) }}在test()方法中找到了Self::abstract_exec()方法
pub(crate) fn abstract_exec( this: &JsObject, input: JsString, context: &mut Context,) -> JsResult<Option<JsObject>> { // 1. Assert: Type(R) is Object. // 2. Assert: Type(S) is String. // 3. Let exec be ? Get(R, "exec"). let exec = this.get("exec", context)?; // 4. If IsCallable(exec) is true, then if let Some(exec) = exec.as_callable() { // a. Let result be ? Call(exec, R, « S »). let result = exec.call(&this.clone().into(), &[input.into()], context)?; // b. If Type(result) is neither Object nor Null, throw a TypeError exception. if !result.is_object() && !result.is_null() { return context.throw_type_error("regexp exec returned neither object nor null"); } // c. Return result. return Ok(result.as_object().cloned()); } // 5. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]). if !this.is_regexp() { return context.throw_type_error("RegExpExec called with invalid value"); } // 6. Return ? RegExpBuiltinExec(R, S). Self::abstract_builtin_exec(this, &input, context)}又在Self::abstract_exec()方法中找到了Self::abstract_builtin_exec()方法
pub(crate) fn abstract_builtin_exec( this: &JsObject, input: &JsString, context: &mut Context,) -> JsResult<Option<JsObject>> { // 1. Assert: R is an initialized RegExp instance. let rx = { let obj = this.borrow(); if let Some(rx) = obj.as_regexp() { rx.clone() } else { return context.throw_type_error("RegExpBuiltinExec called with invalid value"); } }; // 2. Assert: Type(S) is String. // 3. Let length be the number of code units in S. let length = input.encode_utf16().count(); // 4. Let lastIndex be ℝ(? ToLength(? Get(R, "lastIndex"))). let mut last_index = this.get("lastIndex", context)?.to_length(context)?; // 5. Let flags be R.[[OriginalFlags]]. let flags = &rx.original_flags; // 6. If flags contains "g", let global be true; else let global be false. let global = flags.contains('g'); // 7. If flags contains "y", let sticky be true; else let sticky be false. let sticky = flags.contains('y'); // 8. If global is false and sticky is false, set lastIndex to 0. if !global && !sticky { last_index = 0; } // 9. Let matcher be R.[[RegExpMatcher]]. let matcher = &rx.matcher; // 10. If flags contains "u", let fullUnicode be true; else let fullUnicode be false. let unicode = flags.contains('u'); // 11. Let matchSucceeded be false. // 12. Repeat, while matchSucceeded is false, let match_value = loop { // a. If lastIndex > length, then if last_index > length { // i. If global is true or sticky is true, then if global || sticky { // 1. Perform ? Set(R, "lastIndex", +0????, true). this.set("lastIndex", 0, true, context)?; } // ii. Return null. return Ok(None); } // b. Let r be matcher(S, lastIndex). // Check if last_index is a valid utf8 index into input. let last_byte_index = match String::from_utf16( &input.encode_utf16().take(last_index).collect::<Vec<u16>>(), ) { Ok(s) => s.len(), Err(_) => { return context .throw_type_error("Failed to get byte index from utf16 encoded string") } }; let r = matcher.find_from(input, last_byte_index).next(); match r { // c. If r is failure, then None => { // i. If sticky is true, then if sticky { // 1. Perform ? Set(R, "lastIndex", +0????, true). this.set("lastIndex", 0, true, context)?; // 2. Return null. return Ok(None); } // ii. Set lastIndex to AdvanceStringIndex(S, lastIndex, fullUnicode). last_index = advance_string_index(input, last_index, unicode); } Some(m) => { // c. If r is failure, then #[allow(clippy::if_not_else)] if m.start() != last_index { // i. If sticky is true, then if sticky { // 1. Perform ? Set(R, "lastIndex", +0????, true). this.set("lastIndex", 0, true, context)?; // 2. Return null. return Ok(None); } // ii. Set lastIndex to AdvanceStringIndex(S, lastIndex, fullUnicode). last_index = advance_string_index(input, last_index, unicode); // d. Else, } else { //i. Assert: r is a State. //ii. Set matchSucceeded to true. break m; } } } }; // 13. Let e be r's endIndex value. let mut e = match_value.end(); // 14. If fullUnicode is true, then if unicode { // e is an index into the Input character list, derived from S, matched by matcher. // Let eUTF be the smallest index into S that corresponds to the character at element e of Input. // If e is greater than or equal to the number of elements in Input, then eUTF is the number of code units in S. // b. Set e to eUTF. e = input.split_at(e).0.encode_utf16().count(); } // 15. If global is true or sticky is true, then if global || sticky { // a. Perform ? Set(R, "lastIndex", ????(e), true). this.set("lastIndex", e, true, context)?; } // 16. Let n be the number of elements in r's captures List. (This is the same value as 22.2.2.1's NcapturingParens.) let n = match_value.captures.len(); // 17. Assert: n < 23^2 - 1. debug_assert!(n < 23usize.pow(2) - 1); // 18. Let A be ! ArrayCreate(n + 1). // 19. Assert: The mathematical value of A's "length" property is n + 1. let a = Array::array_create(n + 1, None, context)?; // 20. Perform ! CreateDataPropertyOrThrow(A, "index", ????(lastIndex)). a.create_data_property_or_throw("index", match_value.start(), context) .expect("this CreateDataPropertyOrThrow call must not fail"); // 21. Perform ! CreateDataPropertyOrThrow(A, "input", S). a.create_data_property_or_throw("input", input.clone(), context) .expect("this CreateDataPropertyOrThrow call must not fail"); // 22. Let matchedSubstr be the substring of S from lastIndex to e. let matched_substr = if let Some(s) = input.get(match_value.range()) { s } else { "" }; // 23. Perform ! CreateDataPropertyOrThrow(A, "0", matchedSubstr). a.create_data_property_or_throw(0, matched_substr, context) .expect("this CreateDataPropertyOrThrow call must not fail"); // 24. If R contains any GroupName, then // 25. Else, let named_groups = match_value.named_groups(); let groups = if named_groups.clone().count() > 0 { // a. Let groups be ! OrdinaryObjectCreate(null). let groups = JsValue::from(JsObject::empty()); // Perform 27.f here // f. If the ith capture of R was defined with a GroupName, then // i. Let s be the CapturingGroupName of the corresponding RegExpIdentifierName. // ii. Perform ! CreateDataPropertyOrThrow(groups, s, capturedValue). for (name, range) in named_groups { if let Some(range) = range { let value = if let Some(s) = input.get(range.clone()) { s } else { "" }; groups .to_object(context)? .create_data_property_or_throw(name, value, context) .expect("this CreateDataPropertyOrThrow call must not fail"); } } groups } else { // a. Let groups be undefined. JsValue::undefined() }; // 26. Perform ! CreateDataPropertyOrThrow(A, "groups", groups). a.create_data_property_or_throw("groups", groups, context) .expect("this CreateDataPropertyOrThrow call must not fail"); // 27. For each integer i such that i ≥ 1 and i ≤ n, in ascending order, do for i in 1..=n { // a. Let captureI be ith element of r's captures List. let capture = match_value.group(i); let captured_value = match capture { // b. If captureI is undefined, let capturedValue be undefined. None => JsValue::undefined(), // c. Else if fullUnicode is true, then // d. Else, Some(range) => { if let Some(s) = input.get(range) { s.into() } else { "".into() } } }; // e. Perform ! CreateDataPropertyOrThrow(A, ! ToString(????(i)), capturedValue). a.create_data_property_or_throw(i, captured_value, context) .expect("this CreateDataPropertyOrThrow call must not fail"); } // 28. Return A. Ok(Some(a))}Self::abstract_builtin_exec()方法中存在global以及last_index这样看来最终执行的方法就是在这里了，仔细查看该方法中的代码（代码写的很详细而且每一步都有注释）
在第 12 步中：
lastIndex 超过文本长度且当 global 存在时将 lastIndex 置为 0
获取匹配到的值（
match_value）如果未匹配到则置为
advance_string_index()方法的返回值advance_string_index()不在当前问题的考虑范围 https://tc39.es/ecma262/#sec-...
第 13 步获取匹配到的值的 endIndex
第 15 步将 lastIndex 置为 endIndex
至此也就整明白了g标志的含义，在正则的原型链中存在一个lastIndex，如果匹配为真时lastIndex不会重置为 0 ，下一次开始时继承了上次位置，
结论
在问题代码中分析
const reg = /[a-z]/g; // 声明后，lastIndex 为 0reg.test('a'); // => true；第一次匹配后，lastIndex 为 1reg.test('a'); // => false；第二次匹配由于 lastIndex 为 1，且字符只有一个，得到 false，将 lastIndex 置为 0reg.test('a'); // => true；下面依次循环前两次的逻辑reg.test('a'); // => false；reg.test('a'); // => true；到此，相信大家对“JavaScript正则表达式中g标志实例分析”有了更深的了解，不妨来实际操作一番吧！这里是编程网网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
