The regexp for a US number as suggested here:
^[+-]?(\d*|\d{1,3}(,\d{3})*)(\.\d+)?\b$
works perfectly fine. I tried several examples using this service https://regex101.com/
However, using tcl's regexp with as follows:
regexp { ^[+-]?(\d*|\d{1,3}(,\d{3})*)(\.\d+)?\b$ } 120.00
returns '0'.
Have I arrived in tcl "Quoting Hell"?