Often the question is asked "What is the notation for <octal/hex/binary/base-ten> numbers in software"?

There are sometimes incomplete or conflicting notations. Please add any other forms.  ''Reformatted as a table for easier comparison. -sf''

'''NOTE:''' not all forms listed below are actually syntactically correct; rather, the forms ''would be'' correct if taken to their extremes and actually implemented.

                                     Intel/                6502/68K
          Raw Form   CeeLanguage      Zilog     Ada/VHDL  assembler  RexxLanguage       Verilog

 Binary   10011101   0b10011101   10011101B   2#10011101  %10011101   '10011101'b   8'b10011101
 Octal           5           05         05O          8#5         @5        '101'b          4'o5
 Hex            1F         0x1F        01FH        16#1F        $1F         '1F'x         8'h1F
 Decimal        27           27          27           27         27            27            27


Footnotes:
* In CeeLanguage/CeePlusPlus; any integer literal starting with 0x is hex, anything starting with 0 is octal.  Decimal integer literals must start with 1-9; fortunately the number "0" is the same whether decimal or hex.  The lexemes for floating point numbers (things with a decimal point in them) are always decimal in C/C++.
* '$' has been used as a hex prefix for many years (AppleTwo assembly, many ForthLanguage implementations)

''There are conflicts. For instance, is "1001" binary or base-ten? Lexical analysis will not be able to resolve the difference, although a LALR syntax analyzer might - with proper context''

Why is there no nice notation for binary. . . or "why doesn't C/C++ recognize 1001B as binary?"

* Dunno; it would be useful, especially given the low-level bit-frobbing that C/C++ are frequently used for.
** Binary literals (0b1001010) are added to many embedded C compilers, such as for PIC microcontrollers, since you do a lot bit-level I/O on these devices.

Some languages have an arbitrary-radix format.  

Verilog, a hardware description language, extends number to include "don't care's" and "high-impedance" values as well as traditional numbers.
 
 <length>'<radix><digits>

Where <length> is the number of bits in the value, written decimal, <radix> is the radix expressed as one of h, d, o, or b, and <digits> is the actual number, written in base <radix>.  0-9 are themselves, a-f represent 10 through 15, z represents hi-impedance, x represents "don't care".  Some examples are
    10'd1023
    3'b1zz
    3'b101
    3'o7
    32'hdead_beef

Other languages have similar constructs.
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''shouldn't the same example value (e.g. 42 2A 52 101010) be used for all bases?''

I interpreted the original idea behind this page as demonstrating syntax, not arithmetic equality.  In fact, I used the same numbers which were originally posted by the original author.  --SamuelFalvo
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See also: NumberTypes
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CategoryMath