regex not working for angular 7 input pattern

Question

What I tried

field: {       
"pattern": "((?!(demo case)).)*" 
}


 <input matInput spellcheck="false" [formControlName]="field.key" [id]="field.key" type="text" [pattern]="field.pattern" >

its working only for demo case , but when I type Demo Case or DEMO case its not working

I am looking to make it as case - insensitive