You would need to use something like \d{5}
which will match 5 digits.
Depending on the language you are using you would then access whatever the group matched.
For instance, in Java:
String str ="AF1400006";
Pattern p = Pattern.compile("\\d{5}");
Matcher m = p.matcher(str);
if(m.find())
System.out.println(m.group());
Which yields the number you are after.
An example of the expression is available here.
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solved Finding first five digits in a var with Regex [closed]