The Regex Coach - Tutorial step 7

Move the start of the string back to position 0 by pressing the first of the four border buttons four times.

Now use your mouse or the keyboard to select the second a in the regular expression.

Screen Shot

The corresponding a in the target string is now marked with an orange background.

Also, the highlight message below the "selection" highlight button now informs you that the part of the target string which matches the a you've selected ranges from position 3 to position 4 (exclusively).