Filtering a list using starting two letter of an string in array

How to filter content using ngSwitch and taking only first two letters of a string? I have tried to solve it myself some how but I’m doing it wrong way I guess.

    <ion-segment scrollable mode="md" [(ngModel)]="filter">
        <ion-segment-button mode="md" class="ce-sm-segment" value="all" (click)="showSelectedLetterPopup('all')">
            <ion-icon name="medical"></ion-icon>
        <ion-segment-button mode="md" class="ce-sm-segment" value="starred" (click)="showSelectedLetterPopup('☆')">
            <ion-icon name="star-outline"></ion-icon>
        <ion-segment-button mode="md" *ngFor="let ltr of twoLettersList" class="ion-text-lowercase" value={{ltr}}
    <div [ngSwitch]="filter.substring(0, 2)" *ngFor="let word of (words ? words : [])">
        <ion-item *ngSwitchCase="filter" routerLink="/tabs/dictionary/word-detail/{{}}">

I have following objects in the page component:

twoLetterArray = ['aa', 'ab', 'ac', 'ad', 'ae']
words = ['Aachen', 'Abaco', 'acetone', 'adaptation', 'Aegeus']