C中常数的正则表达式

Question

I want to write regular expression for constants in C language. So I tried this:

Let

• digit -> 0-9,
• digit_oct -> 0-7,
• digit_hex -> 0-9 | af | AF

Then:

• RE = digit+ U 0digit_oct+ U 0xdigit_hex+

I want to know whether I have written correct RE Is there any other way of writing this?

Answer 1

There is another type of integer constants, namely integer character constants such as 'a' or '\\n' . In C99 these are constants and their type is just int .

The best regular expressions for all these are found in the standard, section 6.4, http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf

Answer 2

The 'RE' makes sense if we interpret the 'U' as being similar to set union. However, it is more conventional to use a '|' symbol to denote alternatives.

First, you are only dealing with integer constants, not with floating point or character or string constants, let alone more complex constants.

Second, you have omitted ' 0X ' as a valid hex prefix.

Third, you have omitted the various suffixes: U , L , LL , ULL (and their lower-case and mixed case synonyms and permutations).

Also, the C standard (§6.4.4.1) distinguishes between digits and non-zero digits in a decimal constant:

decimal-constant:
    nonzero-digit
    decimal-constant digit

Any integer constant starting with a zero is an octal constant, never a decimal constant. In particular, writing 0 is writing an octal constant.

Answer 3

First, C does not support Unicode literals, so you can eliminate the last rule. You also only define integer literals, not floating-point literals and not string or character literals. For the sake of my convenience I assume that that is what you intended.

INT    := OCTINT | DECINT | HEXINT
DECINT := [1-9] [0-9]* [uU]? [lL]? [lL]?
OCTINT := 0 [0-7]* [uU]? [lL]? [lL]?
HEXINT := 0x [0-9a-fA-F]+ [uU]? [lL]? [lL]?

These only describe the form of the literals, not any logic such as maximum values.

Answer 4

From perl point of view I came up with the following regexp, after reading ISO C 2011:

my $I_CONSTANT = qr/^(?:(0[xX][a-fA-F0-9]+(?:[uU](?:ll|LL|[lL])?|(?:ll|LL|[lL])[uU]?)?)             # Hexadecimal
                      |([1-9][0-9]*(?:[uU](?:ll|LL|[lL])?|(?:ll|LL|[lL])[uU]?)?)                    # Decimal
                      |(0[0-7]*(?:[uU](?:ll|LL|[lL])?|(?:ll|LL|[lL])[uU]?)?)                        # Octal
                      |([uUL]?'(?:[^'\\\n]|\\(?:[\'\"\?\\abfnrtv]|[0-7]{1..3}|x[a-fA-F0-9]+))+')    # Character
                    )$/x;