1)  Java identifier:

A java identifier can contain letters, digits, underscores, and $.
It must START with a non-digit.

[A-Za-z_$]		// first character of variable

[\w$]*			// the remainder of the characters

Full expression:

[A-Za-z_$][\w$]*	// one way to write it.

[\w$&&[^\d]][\w$]*	// another way to write it.

-------------------

2)  Clock time:

Hours value can be a 1-digit number (non-zero):		[1-9]
OR
it can be a 10, 11, or 12:				1[0-2]

Minutes can be a 2-digit number from 00-59:		[0-5][0-9]
						(or)	[0-5]\d

AM or PM can easily be expressed like this:		[ap]m

So the full expression:

([1-9]|(1[0-2])):[0-5]\d [ap]m

-----------------------------

3) IP address  (trickier)

Form is A.B.C.D, where each of A, B, C, D are values from 0 - 255

1-digit number:		[0-9]		or		\d
2-digit number:		[1-9][0-9]	or		[1-9]\d	
100-199:		1[0-9][0-9]	or		1\d{2}
200-249:		2[0-4]\d
250-255:		25[0-5]

OR all of these together to get the value of one of the number positions

\d|([1-9]\d)|(1\d{2})|(2[0-4]\d)|(25[0-5])

The period needs to be escaped (because it has its own meaning as a 
regular expression pre-defined character class otherwise)	\.

Now, do the above expression with a period following it.  Repeated 3x.  
Then the above expression once more:

((\d|([1-9]\d)|(1\d{2})|(2[0-4]\d)|(25[0-5]))\.){3}	// first 3x

Final expression:

((\d|([1-9]\d)|(1\d{2})|(2[0-4]\d)|(25[0-5]))\.){3}(\d|([1-9]\d)|(1\d{2})|(2[0-4]\d)|(25[0-5]))