Regular Expression For Decimal Value
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I want a regular expression for decimal input, which can take 4 digits before decimal or comma and 2 digits after decimal/comma. 5 digit number without decimal is invalid. Also can accept null.

Correct Values can be:

1, 1111, 2222.56, 0.99, 9999.5 


Invalid

88888, 8.888,

Validation show also include comma along with decimal. 

If possible, then also explain the expression.

Thanks