The RegEx

// Requires a decimal and commas
^\$?(([1-9]\d{0,2}(,\d{3})*)|0)?\.\d{1,2}$

// Allows a decimal, requires commas
(?=.*\d)^\$?(([1-9]\d{0,2}(,\d{3})*)|0)?(\.\d{1,2})?$

// Decimal and commas optional
(?=.*\d)^\$?(([1-9]\d{0,2}(,\d{3})*)|0)?(\.\d{1,2})?$

// Decimals required, commas optional
^\$?(([1-9]\d{0,2}(,\d{3})*)|0)?\.\d{1,2}$

// *Requires/allows X here also implies "used correctly"

The RegEx Breakdown

• When the optional parts are too liberal, we need to look ahead and guarantee there's a number: (?=.*\d)
• May or may not start with a dollar sign (I assume negatives are invalid): ^\$?
  • Follow that with -? to allow negative numbers
• Begins with 1-3 numbers: [1-9]\d{0,2}
  • Could almost be (\d{1,3}), but that would allow "0,123" _ One exception, can start with 0 in the case of "$0.50" or "0.50": |0 _ These regexes assume multiple leading 0's are invalid
• Any number of three digit numbers separated by comma: (,\d{3})*
  • Remove ? before \. if you want to disallow numbers starting with "$."
• Requires or allows decimal (one or two digits): \.\d{1,2} or (\.\d{1,2})? respectively _ End with $ (unescaped) to make sure there's nothing after a valid number (like $1,000.00b)

To use the regex, use the string's match method and encase the regex between two forward slashes.

// The return will either be your match or null if not found
yourNumber.match(/(?=.)^\$?(([1-9][0-9]{0,2}(,[0-9]{3})*)|0)?(\.[0-9]{1,2})?$/);

// For just a true/false response
!!yourNumber.match(/(?=.)^\$?(([1-9][0-9]{0,2}(,[0-9]{3})*)|0)?(\.[0-9]{1,2})?$/);