您可以使用此正则表达式：

/^[A-Za-z]+(?:\s+[A-Za-z]+)*(?=:)/gm

RegEx Demo

RegEx细分：

• ^：开始
• [A-Za-z]+：匹配1个以上的英语字母
• (?:\s+[A-Za-z]+)*：匹配1+个空格，后跟1+个英语字母。重复该组0次或更多次。
• (?=:)：LookaheadAssert在下一个位置有:

尝试此版本：

let text = `Necessity:(noun)the need for something, or something that is needed
Example: In my work, a computer is a necessity.

Lay of the land: (idiom n. Or v.)the general state or condition of affairs under consideration; the facts of a situation
Example: We asked a few questions to get the lay of the land.

Mumble:(verb) to speak quietly or in an unclear way so that the words are difficult to understand
Example: She mumbled something about needing to be home, then left.
`;

let matches = text.match(/(^|(?<=\n\n))\w+(?: \w+)*(?=:)/g);

console.log(matches);

下面是对所用正则表达式模式的解释：

• (
• ^字符串的开头
• |或
• (?<=\n\n) lookbehind并在前面Assert两个换行符
• )
• \w+匹配单词
• (?: \w+)*后跟空格和另一个单词，零次或多次
• (?=:)Assert:跟在这个或这些字后面

我喜欢我的正则表达式尽可能短，所以这是我的看法。

const text = `Necessity:(noun)the need for something, or something that is needed Example: In my work, a computer is a necessity.

Lay of the land: (idiom n. Or v.)the general state or condition of affairs under consideration; the facts of a situation Example: We asked a few questions to get the lay of the land

Mumble:(verb) to speak quietly or in an unclear way so that the words are difficult to understand Example: She mumbled something about needing to be home, then left.`;

let matches = text.match(/^\S[^:]+/gm);
console.log(matches);

细目分类：

• ^置位行首
• \S匹配任何非空白字符。
• [^:]+匹配不是:的所有字符一次或无限次。

在模式[A-Za-z]+(?=\:\S)中，如果有多个单词，则不匹配任何空格，并且使用(?=\:\S)Assert右边的冒号后跟非空格字符
如果应该有一个带有冒号和括号的部分跟随并使用捕获组：

^([A-Za-z].*?):\s*\([^()]*\)

说明

• ^字符串开头
• ([A-Za-z].*?)捕获组1，尽可能匹配字符A-Za-z后跟除换行符以外的任何字符
• :\s*匹配冒号和可选空格字符
• \([^()]*\)与(...)匹配

请参见regex demo。

const regex = /^([A-Za-z].*?):\s*\([^()]*\)/gm;
const str = `Necessity:(noun)the need for something, or something that is needed Example: In my work, a computer is a necessity.

Lay of the land: (idiom n. Or v.)the general state or condition of affairs under consideration; the facts of a situation Example: We asked a few questions to get the lay of the land

Mumble:(verb) to speak quietly or in an unclear way so that the words are difficult to understand Example: She mumbled something about needing to be home, then left.`;

const result = Array.from(str.matchAll(regex), m => m[1]);
console.log(result);

您可以按照上面的方法进行操作：

let regex_exp = /(^|(?<=\n\n))\w+(?: \w+)*(?=:)/g;
let matches = text.match(regex_exp);
console.warn(matches);