/^
    [+-]?            # first, match an optional sign
    (                # then match integers or f.p. mantissas:
        \d+.\d+      # mantissa of the form a.b
        |\d+.        # mantissa of the form a.
        |\.\d+       # mantissa of the form .b
        |\d+         # integer of the form a
    )
    ([eE][+-]?\d+)?  # finally, optionally match an exponent
$/x;