Maybe slightly cleaner ? 

(123456.56567 * 4) =~ s{ \d+ }{ $& =~ s/ \B (?= (?:\d{3})+ $ ) /,/xgr 
+}xer

[download]
outputs 
493,826.26268

[download]

In reply to Re^5: regex in REPLACEMENT in s/// by tybalt89
in thread regex in REPLACEMENT in s/// by perlboy_emeritus

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