在Javascript中有几种（有时是棘手的）解决方案用于lookbehind正则表达式。但是，这是最简单的方法，如果我需要零宽度！在全局搜索后面查看表达式，这可能会重叠。例如。使用/(?<=[01])\d/g我们可以执行以下操作：
let a = "--1--01001--1087---";
let result = a.replace(/(?<=[01])\d/g, "#");
// This should print "--1--0####--1##7---" if lookbehind would be supported
console.log(result);
或者另一个例子：我们如何创建一个\b之类的表达式，它只适用于字母（[a-zA-Z]）。 （Lookforward毫无疑问。只是JS不支持lookbehind）。
答案 0 :(得分：1)
通过逆转方式的后视似乎是最简单的。 对于像这样的短数字模式，这种方法最好。
function revStr(str) {
return str.split('').reverse().join('');
}
var s = "--1--01001--1087---";
var rxp = /\d(?=[01])/g;
var result = revStr(revStr(s).replace(rxp, "#"));
document.write(result);
逻辑：
\d(?=[01])是(?<=[01])\d revStr(s)函数注意：
如果你需要在JavaScript中使用可变宽度的后视和前瞻，我可以推荐阅读Steven Levithan撰写的JavaScript Regex Lookbehind Redux文章，在那里你可以找到示例函数如何使用XRegExp实现该行为。以下是功能：
// Simulating infinite-length leading lookbehind in JavaScript. Uses XRegExp.
// Captures within lookbehind are not included in match results. Lazy
// repetition in lookbehind may lead to unexpected results.
(function (XRegExp) {
function prepareLb(lb) {
// Allow mode modifier before lookbehind
var parts = /^((?:\(\?[\w$]+\))?)\(\?<([=!])([\s\S]*)\)$/.exec(lb);
return {
// $(?!\s) allows use of (?m) in lookbehind
lb: XRegExp(parts ? parts[1] + "(?:" + parts[3] + ")$(?!\\s)" : lb),
// Positive or negative lookbehind. Use positive if no lookbehind group
type: parts ? parts[2] === "=" : !parts
};
}
XRegExp.execLb = function (str, lb, regex) {
var pos = 0, match, leftContext;
lb = prepareLb(lb);
while (match = XRegExp.exec(str, regex, pos)) {
leftContext = str.slice(0, match.index);
if (lb.type === lb.lb.test(leftContext)) {
return match;
}
pos = match.index + 1;
}
return null;
};
XRegExp.testLb = function (str, lb, regex) {
return !!XRegExp.execLb(str, lb, regex);
};
XRegExp.searchLb = function (str, lb, regex) {
var match = XRegExp.execLb(str, lb, regex);
return match ? match.index : -1;
};
XRegExp.matchAllLb = function (str, lb, regex) {
var matches = [], pos = 0, match, leftContext;
lb = prepareLb(lb);
while (match = XRegExp.exec(str, regex, pos)) {
leftContext = str.slice(0, match.index);
if (lb.type === lb.lb.test(leftContext)) {
matches.push(match[0]);
pos = match.index + (match[0].length || 1);
} else {
pos = match.index + 1;
}
}
return matches;
};
XRegExp.replaceLb = function (str, lb, regex, replacement) {
var output = "", pos = 0, lastEnd = 0, match, leftContext;
lb = prepareLb(lb);
while (match = XRegExp.exec(str, regex, pos)) {
leftContext = str.slice(0, match.index);
if (lb.type === lb.lb.test(leftContext)) {
// Doesn't work correctly if lookahead in regex looks outside of the match
output += str.slice(lastEnd, match.index) + XRegExp.replace(match[0], regex, replacement);
lastEnd = match.index + match[0].length;
if (!regex.global) {
break;
}
pos = match.index + (match[0].length || 1);
} else {
pos = match.index + 1;
}
}
return output + str.slice(lastEnd);
};
}(XRegExp));
这些函数中的每一个都有三个参数：要搜索的字符串，作为字符串的lookbehind模式（可以使用XRegExp语法扩展）和主正则表达式。 XRegExp.replaceLb为替换值采用第四个参数，可以是字符串或函数。
用法示例如下：
XRegExp.execLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", XRegExp("(?i)(?<first>c)at"));
// -> ["cat", "c"]
// Result has named backref: result.first -> "c"
XRegExp.execLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> null
XRegExp.testLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i);
// -> true
XRegExp.testLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> false
XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i);
// -> 18
XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> 0
XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<=fluffy\\W+)", /cat\w*/i);
// -> ["cats"]
XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<!fluffy\\W+)", /cat\w*/i);
// -> ["Catwoman", "cats"]
XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<=fluffy\\W+)", /cat/ig, "dog");
// -> "Catwoman's fluffy dog is a cat"
XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, "dog");
// -> "dogwoman's fluffy cat is a dog"
XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, function ($0) {
var first = $0.charAt(0);
return first === first.toUpperCase() ? "Dog" : "dog";
});
// -> "Dogwoman's fluffy cat is a dog"