Regular expression : match either of two conditions?

In your regex, the two alternative branches are anchored separately:

• (^([a-zA-Z]){2}([0-9]){6}) – 2 letters and 6 digits at the start of the string
• | – or
• ([0-9]*)?$ – optional zero or more digits at the end of the string

You need to adjust the boundaries of the group:

data-ng-pattern="/^([a-zA-Z]{2}[0-9]{6}|[0-9]{8})?$/"
                   ^                         ^^^^ 

See the regex demo.

Now, the pattern will match:

• ^ – start of string
• ( – start of the grouping:
  • [a-zA-Z]{2}[0-9]{6} – 2 letters and 6 digits
  • | – or
  • [0-9]{8} – 8 digits
• )? – end of the grouping and ? quantifier makes it match 1 or 0 times (optional)
• $ – end of string.