(defun parse-c-integer (string &key (start 0) (end nil))
  (match-case (string :start start :end end)
    ("0[0-7]*"
     (parse-integer $$ :radix 8))
    ("[0-9]+"
     (parse-integer $$ :radix 10))
    ("0[xX]([0-9A-Fa-f]+)"
     ;; note the submtach addressing $1
     (parse-integer $1 :radix 16))
    (otherwise
     (error "Funny integer: ~S" (subseq string start end)))))

;; Code you get:

#+NIL
(LET ((STRING.537872 STRING))
  (MULTIPLE-VALUE-BIND (CAT S.537868 E.537870 S.537869 E.537871)
      ((LAMBDA (BUFFER.1 &KEY ((:START START) 0) ((:END END) NIL))
         (CHECK-TYPE BUFFER.1 SIMPLE-STRING)
         (LOCALLY (DECLARE (TYPE SIMPLE-STRING BUFFER.1))
           (LET ((LENGTH.2 (LENGTH BUFFER.1)))
             (DECLARE (TYPE (INTEGER 0 72057594037927936) LENGTH.2))
             (SETQ END (OR END LENGTH.2))
             (CHECK-TYPE START (INTEGER 0 72057594037927936))
             (CHECK-TYPE END (INTEGER 0 72057594037927936))
             (ASSERT (<= 0 START END LENGTH.2)))
           (LET ((BUFFER.1 BUFFER.1) (P START) (END END) (C.3 0))
             (DECLARE (TYPE SIMPLE-STRING BUFFER.1) (TYPE (INTEGER 0 72057594037927936) P END START) (TYPE (INTEGER -2 (1114112)) C.3)
                      (IGNORABLE C.3) (IGNORABLE BUFFER.1 P END) (OPTIMIZE (SPEED 3) (SAFETY 0)))
             (PROG ((WON.0 NIL) (R4 -1) (R-1 -1) (R0 -1) (R1 -1) (R2 -1) (R3 -1) (R8 -1))
                     (DECLARE (IGNORABLE WON.0 R4 R-1 R0 R1 R2 R3 R8)
                              (TYPE (INTEGER -1 72057594037927936) R4 R-1 R0 R1 R2 R3 R8))
                     START (GO Q4)
                     SCAN-LOOP (RETURN (VALUES))
                     Q3 (GO Q1)
                     Q4 (WHEN (= P END) (GO Q1))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (COND ((= C.3 48) (PSETQ R8 P) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q7))
                           ((<= 48 C.3 57) (PSETQ R8 P) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q6)))
                     (GO Q1)
                     Q5 (GO Q1)
                     Q6 (WHEN (= P END)
                          (PROGN (PSETQ R3 -1 R2 -1 R1 P R0 R8 R-1 1) (GO Q2)))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (WHEN (<= 48 C.3 57) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q6))
                     (GO Q1)
                     Q7 (WHEN (= P END)
                          (PROGN (PSETQ R3 -1 R2 -1 R1 P R0 (THE (INTEGER 0 72057594037927936) (- P 1)) R-1 0)
                                 (GO Q2)))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (COND ((OR (= C.3 88) (= C.3 120)) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q9))
                           ((OR (<= 48 C.3 55) (= C.3 88) (= C.3 120)) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q8))
                           ((OR (<= 48 C.3 57) (= C.3 88) (= C.3 120)) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q6)))
                     (GO Q1)
                     Q8 (WHEN (= P END)
                          (PROGN (PSETQ R3 -1 R2 -1 R1 P R0 R8 R-1 0) (GO Q2)))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (COND ((<= 48 C.3 55) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q8))
                           ((<= 48 C.3 57) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q6)))
                     (GO Q1)
                     Q9 (WHEN (= P END) (GO Q1))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (WHEN (OR (<= 48 C.3 57) (<= 65 C.3 70) (<= 97 C.3 102))
                       (PSETQ R4 P)
                       (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1)))
                       (GO Q10))
                     (GO Q1)
                     Q10 (WHEN (= P END)
                           (PROGN (PSETQ R3 P R2 R4 R1 P R0 R8 R-1 2) (GO Q2)))
                     (SETQ C.3 (CHAR-CODE (SCHAR BUFFER.1 P)))
                     (WHEN (OR (<= 48 C.3 57) (<= 65 C.3 70) (<= 97 C.3 102)) (SETQ P (THE (INTEGER 0 72057594037927936) (+ P 1))) (GO Q10))
                     (GO Q1)
                     Q1 (GO SCAN-LOOP)
                     Q2 (RETURN (VALUES (LET ((G4 R-1)) (IF (EQL G4 -1) NIL G4))
                                        (LET ((G4 R0)) (IF (EQL G4 -1) NIL G4))
                                        (LET ((G4 R1)) (IF (EQL G4 -1) NIL G4))
                                        (LET ((G4 R2)) (IF (EQL G4 -1) NIL G4))
                                        (LET ((G4 R3)) (IF (EQL G4 -1) NIL G4))))))))
       STRING.537872 :START START :END END)
    (DECLARE (IGNORABLE S.537868 E.537870 S.537869 E.537871))
    (WITH-SUBMATCH-MACROS-2
        (STRING.537872)
        ((0 T S.537868 E.537870) (1 S.537869 S.537869 E.537871))
      (CASE CAT
        ((0) (PARSE-INTEGER $$ :RADIX 8))
        ((1) (PARSE-INTEGER $$ :RADIX 10))
        ((2) (PARSE-INTEGER $1 :RADIX 16))
        (OTHERWISE (ERROR "Funny integer: ~S" (SUBSEQ STRING START END)))))))