You could write the pattern as:

^(?:\d{7,8}|[TD]\d{6,7}|TD\d{5,6})$

Explanation

• ^ Start of string
• (?: Non capture group for the alternatives
  • \d{7,8} Match 7-8 digits
  • | Or
  • [TD]\d{6,7} Match either T or D and 6-7 digits
  • | Or
  • TD\d{5,6} Match TD and 5-6 digits
• ) Close the non capture group
• $ End of string

Regex demo.

const r = /^(?=.{7,8}$)T?D?\d+$/

is the simplest solution. Here's a breakdown of what happens:

import {lookAhead, maybe, sequence, suffix} from "compose-regexp"

const r = sequence(
  // start anchor
  /^/,
  // are there exactly 7 or 8 characters before the end?
  lookAhead(suffix([7,8], /./), /$/),
  // optionally match a 'T'
  maybe('T'),
  // optionally match a 'D'
  maybe('D'),
  // match numbers until the end
  suffix('+', /\d/),
  /$/
)

You can try it out here

With tests aplenty.

I actually figure it out right after posted this question. Simply added |D after T| so like this.

/^(T|D|[0-9]){1}(D|[0-9]){1}([0-9]){5,6}$/