javascript – What’s the regex pattern that comma separates a number’s integer part every 3 digits and not the decimal part?

You can replace (zero-width) matches of the following regular expression with a comma:

\B(?:(?=(?:\d{3})+\.)|(?!.*\.)(?<!\..*)(?=(?:\d{3})+$))

Demo

This expression can be broken down as follows.

\B           # match a non-word boundary
(?:          # begin a non-capture group
  (?=        # begin a positive lookahead 
    (?:      # begin a non-capture group
      \d{3}  # match 3 digits
    )        # end non-capture group
    +        # execute preceding non-capture group >= 1 times 
    \.       # match '.'
  )          # end positive lookahead
  |          # or
  (?!        # begin a negative lookahead
    .*\.     # match >= 0 chars other than line terms then '.'
  )          # end negative lookahead
  (?<!       # begin a negative lookbehind
  \..*       # match '.' then >= 0 chars other than line terms
  )          # end negative lookbehind
  (?=        # begin a positive lookahead
    (?:      # begin a non-capture group
      \d{3}  # match 3 digits
    )        # end non-capture group
    +        # execute preceding non-capture group >= 1 times
    $        # match end of string
  )          # end positive lookahead
)            # end non-capture group

Notice that the first part of the alternation matches string representations of numbers with a decimal point. That way the relatively expensive negative lookbehind ((?<!\..*)) need not be executed when the string contains a decimal point.