您可以使用\$[^$]+来匹配后跟其他字符的$。

let s = '$command this my first text $ this is my second text $ this is my third text $ this is my fourth text';
let res = s.match(/(?:\$[^$]+){2}\$([^$]+)/)[1].trim();
console.log(res);

使用捕获组：

^(?:[^$]*\$){3}\s*([^$]*[^\s$])

说明

• ^字符串开始
• (?:非捕获组作为整个部分重复
• [^$]*\$匹配$以外的可选字符，然后匹配$
• ){3}关闭非捕获组并重复3次
• \s*匹配可选空白字符
• (捕获组1
• [^$]*匹配$以外的可选字符
• [^\s$]匹配$以外的非空白字符
• )关闭组1

参见regex demo。

const s = "$command this my first text $ this is my second text $ this is my third text $ this is my fourth text";
const regex = /^(?:[^$]*\$){3}\s*([^$]*[^\s$])/;
const m = s.match(regex);
if (m) console.log(m[1])

如果是supported，则使用lookbehindAssert仅获取匹配：

const s = "$command this my first text $ this is my second text $ this is my third text $ this is my fourth text";
const regex = /(?<=^(?:[^$]*\$){3}\s*)[^\s$](?:[^$]*[^\s$])?/;
const m = s.match(regex);
if (m) console.log(m[0])

let s = '$command this my first text $ this is my second text $ this is my third text $ this is my fourth text';
let res = s.match(/(?<=((?<=\$).*?\$){2}) this is my.*?(?=\$)/)[0];
console.log(res);

库应该接受正向前看和向后看，因此您可以尝试以下操作：

(?<=((?<=\$).*?\$){2}) this is my.*?(?=\$)