quotes, C (clang) - rextester

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#include <stdio.h>
#include <ctype.h>

// Предполагает, что в dest как минимум столько же места, что и в src.
static void unescape_it_right(char *dest, const char *src)
{
    int c;
    while ((c = *src++) != '\0') {
        if (c != '\\' || !*src) {
            *dest++ = c;   
            continue;
        }      
        
        c = *src++;
        switch (c) {
            case 'a': *dest++ = '\a'; break;
            case 'b': *dest++ = '\b'; break;
            case 'f': *dest++ = '\f'; break;
            case 'n': *dest++ = '\n'; break;
            case 'r': *dest++ = '\r'; break;
            case 't': *dest++ = '\t'; break;
            case 'v': *dest++ = '\v'; break;
            case '0': *dest++ = '\0'; return; // Ибо нехуй.
            case '\'': *dest++ = '\''; break;
            case '"': *dest++ = '"'; break;
            case '\\': *dest++ = '\\'; break;
            case '\?': *dest++ = '\?'; break;
            
            case 'x': {
                static const char xvalue[] = {
                    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0,
                    10, 11, 12, 13, 14, 15,
                };
                if (isdigit(src[0]) && isdigit(src[1])) {                   
                    *dest++ = xvalue[toupper(src[0]) - '0'] << 4 | xvalue[toupper(src[1]) - '0'];
                    src += 2;
                    break;
                }
                // FALLTHROUGH (invalid hex sequence).
            }
            default:
                *dest++ = '\\';
                *dest++ = c;
                break;
        }
    }
    *dest = '\0';
}

int main(void)
{
    char x[] = "xx\\\\xxx\\ty\\r\\nhello\\x22\\xf";
    printf("x before: %s\n", x);
    unescape_it_right(x, x);
    printf("x after: %s", x);
}

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Compilation time: 0.24 sec, absolute running time: 0.23 sec, cpu time: 0.15 sec, memory peak: 3 Mb, absolute service time: 0,48 sec