1. Mandatory sign, integer, fraction, and exponent
^[-+][0-9]+\.[0-9]+[eE][-+]?[0-9]+$

2. Mandatory sing, integer, and fraction, but no exponent
^[-+][0-9]+\.[0-9]+$

3. Optional sign, mandatory integer and fraction, and no exponent
^[-+]?[0-9]+\.[0-9]+$

4. Optional sign and integer, mandatory fraction, and no exponent
^[-+]?[0-9]*\.[0-9]+$

5. Optional sign, integer, and fraction. 
If the integer part is omitted, the fraction is mandatory.
If the fraction is omitted, the decimal dot must be omitted, too. No exponent
^[-+]?([0-9]+(\.[0-9]+)?|\.[0-9]+)$

6. Optional sign, integer, and fraction. 
If the integer part is omitted, the fraction is mandatory.
If the fraction is omitted, the decimal dot is optional. No exponent
^[-+]?([0-9]+(\.[0-9]*)?|\.[0-9]+)$

7. Optional sign, integer, and fraction. 
If the integer part is omitted, the fraction is mandatory.
If the fraction is omitted, the decimal dot must be omitted, too. Optional exponent
^[-+]?([0-9]+(\.[0-9]+)?|\.[0-9]+)([eE][-+]?[0-9]+)?$

8. Optional sign, integer, and fraction. 
If the integer part is omitted, the fraction is mandatory.
If the fraction is omitted, the decimal dot is optional. Optional exponent
^[-+]?([0-9]+(\.[0-9]*)?|\.[0-9]+)([eE][-+]?[0-9]+)?$

9. find float number in a larger body of text
[-+]?(\b[0-9]+(\.[0-9]+)?|\.[0-9]+)([eE][-+]?[0-9]+\b)?