Decide a C integer literal

Retina 0.8.2, 60 59 bytes

i`^(0[0-7]*|0x[\da-f]+|[1-9]\d*)(u)?(l)?(?-i:\3?)(?(2)|u?)$

Try it online! Link includes test cases. Edit: Saved 1 byte thanks to @FryAmTheEggMan. Explanation:

i`

Match case-insensitively.

^(0[0-7]*|0x[\da-f]+|[1-9]\d*)

Start with either octal, hex or decimal.

(u)?

Optional unsigned specifier.

(l)?

Optional length specifier.

(?-i:\3?)

Optionally repeat the length specifier case sensitively.

(?(2)|u?)$

If no unsigned specifier yet, then another chance for an optional specifier, before the end of the literal.

Perl 5 -p, 65 61 bytes

@NahuelFouilleul shaved 4 bytes

$_=/^(0[0-7]*|0x\p{Hex}+|[1-9]\d*)(u?l?l?|l?l?u?)$/i*!/lL|Ll/

Try it online!

Java 8 / Scala polyglot, 89 79 bytes

s->s.matches("(?!.*(Ll|lL))(?i)(0[0-7]*|[1-9]\\d*|0x[\\da-f]+)(u?l?l?|l?l?u?)")

-10 bytes thanks to @NahuelFouilleul

Try it online in Java 8.
Try it online in Scala (except with => instead of -> - thanks to @TomerShetah).

Explanation:

s->           // Method with String parameter and boolean return-type
  s.matches(  //  Check whether the input-string matches the regex
    "(?!.*(Ll|lL))(?i)(0[0-7]*|[1-9]\\d*|0x[\\da-f]+)(u?l?l?|l?l?u?)")

Regex explanation:

In Java, the String#matches method implicitly adds a leading and trailing ^...$ to match the entire string, so the regex is:

^(?!.*(Ll|lL))(?i)(0[0-7]*|[1-9]\d*|0x[\da-f]+)(u?l?l?|l?l?u?)$

 (?!         )     # The string should NOT match:
^   .*             #   Any amount of leading characters
      (     )      #   Followed by:
       Ll          #    "Ll"
         |lL       #    Or "lL"
                   # (Since the `?!` is a negative lookahead, it acts loose from the
                   #  rest of the regex below)

 (?i)              # Using case-insensitivity,
^    (             # the string should start with:       
       0           #   A 0
        [0-7]*     #   Followed by zero or more digits in the range [0,7]
      |            #  OR:
       [1-9]       #   A digit in the range [1,9]
            \d*    #   Followed by zero or more digits
      |            #  OR:
       0x          #   A "0x"
         [     ]+  #   Followed by one or more of:
          \d       #    Digits
            a-f    #    Or letters in the range ['a','f'] 
     )(            # And with nothing in between,
              )$   # the string should end with:
        u?         #   An optional "u"
          l?l?     #   Followed by no, one, or two "l"
       |           #  OR:
        l?l?       #   No, one, or two "l"
            u?     #   Followed by an optional "u"