micropython/extmod/re1.5/charclass.c

#include "re1.5.h"

int _re1_5_classmatch(const char *pc, const char *sp)
{
    // pc points to "cnt" byte after opcode
    int cnt = *pc++;
    while (cnt--) {
        if (!(*sp >= *pc && *sp <= pc[1])) return 0;
    }
    return 1;
}