I want to write a program changing the notation of all hexadecimal numbers found in an assembly source file from traditional (h) to C-style (0x). I have started the coding part but am not sure how can I detect the hexadecimal numbers and eventually change the style and save it back in the file...

I have started writing the program..

## Mips program  - 

.data
fin:   .ascii ""      # filename for input
msg0:   .asciiz "aaaa"
msg1:   .asciiz "Please enter the input file name:"
buffer: .asciiz ""
.text

#-----------------------
    li $v0, 4
    la $a0, msg1
    syscall

    li $v0, 8
    la $a0, fin
    li $a1, 21
    syscall

    jal fileRead            #read from file
    move $s1, $v0           #$t0 = total number of bytes

    li $t0, 0   # Loop counter

loop:
    bge $t0, $s1, end           #if end of file reached OR if there is an error in the file
    lb $t5, buffer($t0)         #load next byte from file

    jal checkhexa       #check for hexadecimal numbers

    addi $t0, $t0, 1            #increment loop counter

j loop

end:

    jal output
    jal fileClose

    li $v0, 10
    syscall

fileRead:
    # Open file for reading
    li   $v0, 13       # system call for open file
    la   $a0, fin      # input file name
    li   $a1, 0        # flag for reading
    li   $a2, 0        # mode is ignored
    syscall            # open a file 
    move $s0, $v0      # save the file descriptor 

    # reading from file just opened
    li   $v0, 14       # system call for reading from file
    move $a0, $s0      # file descriptor 
    la   $a1, buffer   # address of buffer from which to read
    li   $a2, 100000   # hardcoded buffer length
    syscall            # read from file

jr $ra

Any help would be appreciated.