lib.c

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
#include <regex.h>

typedef int8_t i8; typedef  uint8_t u8; typedef  int16_t i16; typedef uint16_t u16; typedef  int32_t i32; typedef uint32_t u32; typedef  int64_t i64; typedef uint64_t u64; typedef double   f64; typedef float    f32; typedef u64 ux;

#define MAX_MATCHES 1024
#define MAX_GROUPS 1024
#define MAX_OUT 1024

i32 regex_c(char* out, char* source, char* regex_str) {
    regex_t regex_compiled;
    regmatch_t group_arr[MAX_GROUPS];
    u64 m=0;
    i32 out_offs=0;
    char* cursor=source;

    i32 out_n=0;
    memset(out, '\0', sizeof(char) * MAX_OUT);

    if (regcomp(&regex_compiled, regex_str, REG_EXTENDED)) {
        printf("Could not compile regular expression.\n");
        return 0;
    };

    while (out_n<MAX_MATCHES) {
        i32 r=regexec(&regex_compiled, cursor, MAX_GROUPS, group_arr, 0);
        u32 offset=0;

        if (r == REG_NOMATCH) { break; }

        for (u32 g=0; g<MAX_GROUPS; g++) {
            if (group_arr[g].rm_so == (size_t)-1) { break;  } /* No more groups */
            if (g == 0) { offset = group_arr[g].rm_eo; }

            i32 n= group_arr[g].rm_eo-group_arr[g].rm_so;
            strncpy(out + out_offs, cursor+ group_arr[g].rm_so, n);
            out[out_offs+n] = 0;
            out_offs+=n + 1;
            out_n++;
       }

        cursor += offset;
    }

    regfree(&regex_compiled);

    return out_n;
}

Makefile

lib.so: lib.c
	gcc -shared lib.c -o lib.so

regex.bqn

regex_c ← "./lib.so" •FFI ⟨"i64","regex_c", "&u8:i8", "*u8:c8", "*u8:c8"⟩ 

Regex←{
  ⟨n, ch⟩← Regex_C ⟨1024⥊0, 𝕩∾@+0, 𝕨∾@+0⟩
  @⊸+¨(ch=0)(⊢-˜(+`׬))⊸⊔ch
}

•Show "[a-z]+" Regex "hello abc world!"