Now I want to put some more restriction that this string should contain something or nothing in double quote in the end. i.e string starting with date then white space then some characters then ending with double quotes where something or nothing can be withing double quotes eg. "OEM:imst2k19:4" or ""

Basically, what you saying is that the rest of the string should contain two double quotes, where the last one ends the string. This part is expressed as:

/[^"]*"[^"]*"$/
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