const regex = /(?>\\caption(?:of)?\b[^{]*{|(?<!^)\G)(?>[^}{]|(\{(?:[^}{]*+|(?-1))*}))*?\K\\{2}/g;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?>\\\\caption(?:of)?\\b[^{]*{|(?<!^)\\G)(?>[^}{]|(\\{(?:[^}{]*+|(?-1))*}))*?\\K\\\\{2}', 'g')
const str = ` \\begin{figure}
\\includegraphics{pic.pdf}
\\caption[]{My caption \\\\
Source: XYZ}
\\label{fig:pic_1}
\\end{figure}
\\begin{figure}[H]
\\includegraphics{pic.pdf}
\\captionof[]{My caption \\\\ xyz \\\\ abc
\\label{fig:pic_1} }
\\end{figure}
\\begin{figure}[H]
\\includegraphics{pic.pdf}
\\caption[]{My caption {with extra brackets}
Source: XYZ}
\\label{fig:pic_1}
\\end{figure}
\\begin{figure}[H]
\\includegraphics{pic.pdf}
\\caption[]{My caption}
\\end{figure}
Some text\\\\ %% This \\\\ should not be changed, it's not within a caption
More text
\\begin{figure}[H]
\\includegraphics{pic.pdf}
\\caption[]{My caption \\\\ Source: XYZ}
\\label{fig:pic_1}
\\end{figure}`;
const subst = `\\par`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions