We have string.isdigit(), why not string.isNumber()?

[MooMaster]

N00b question alert! I did a search for isdigit() in the group
discussion, and it didn't look like the question had been asked in the
first 2 pages, so sorry if it was...

The manual documentation says:
"isdigit( )

Return true if all characters in the string are digits and there is at
least one character, false otherwise.
For 8-bit strings, this method is locale-dependent. "

So it makes sense that something like 5.6 would return false. But what
if we want to make sure that our string is a valid number, ie decimals
included?

I know how to write a regexp or method or whatever to do this, my main
question is *why* something like an isNumber() method is not baked
into the class. Does such functionality exist somewhere else in the
standard library that I'm just missing?