Regex is returning wrong results

Question

I've the below code:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    // First part 
    s := "Never say never."
    r := regexp.MustCompile(`(?i)^n`)  // Do we have an 'N' or 'n' at the beginning?
    fmt.Printf("%v", r.MatchString(s)) // true, case insensitive, correct answer
    fmt.Println()

    // Second part 
    r = regexp.MustCompile(`^a.b$`)
    matched := r.MatchString("aaxbb")
    fmt.Println(matched)  // false, why?
    fmt.Println()

    // Third part 
    re := regexp.MustCompile(`.(?P<Day>\d{2})`)
    matches := re.FindStringSubmatch("Some random date: 2001-01-20")
    dayIndex := re.SubexpIndex("Day")
    fmt.Println(matches[dayIndex]) // 20, correct answer
    fmt.Println()

    // Fourth part 
    re = regexp.MustCompile(`^card\s.\s(?P<Day>\d{2})`)
    matches = re.FindStringSubmatch("card Some random date: 2001-01-20")
    dayIndex = re.SubexpIndex("Day")
    fmt.Println(matches[dayIndex]) // panic: runtime error: index out of range [1] with length 0, why?
}

First and Third parts are ok, and returning correct answers. Second and Fourth parts are wrong and returns either wrong answers or errors, though they are almost same as parts 1 and 3 but with slight tuning?