Simulate `grep -C 3 regex'

BEGIN {
    lines = 3
    buffer = lines + 1
    n = lines + 2
    i = j = 0
    separate = 2
}

/regex/ {
    if (lines && separate == 1)
        print "--"
    while (i != j) {
        print a[i++]
        if (i == buffer)
            i = 0
    }
    n = separate = 0
}

n > lines {
    a[j++] = $0
    if (j == buffer)
       j = 0
    if (i == j) {
        i++
        if (i == buffer)
            i = 0
        if (!separate)
            separate++
    }
    next
}

n++ <= lines