In Overleaf or maybe another LaTeX editor, if like me you only used \cite{...} instead of ~\cite{...} and your advisor cracks down on you because it should be with a space before de square brackets [1] and not the other way[2], fear no more.

The search and replace tool of Overleaf leverages all the power of regexes. Lets add a ~ character before all the \cite that do not have one yet.

Search : ([^~])\\cite (do not forget to select the RegExp Search option)

Replace by : $1~\cite

Click Replace to check that I do not misguide you by only fixing one occurence at a time or be over confident in the power of regexes and click "All".

Done.