CAD tip # 4136:

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The search/replace expression for thousands separator (",") can be e.g.:

-?\([0-9]+\)\([0-9][0-9][0-9]\)
\1,\2

1234 -> 1,234
-1234.5 -> -1,234.5

Update: the newer version of the command - SRXTEXT2 - has easier, VBscript-like formatting of the control codes.

For this thousands separator task, you can use the following, more flexible search/replace code:

(\d)(?=(\d{3})+\b)
$1,