Find pattern with regexp doesn't allow to find patterns like "A{4,6}"

Scenario to reproduce:

1. Open "data/samples/FASTA/human_T1.fa".
2. Open "Find pattern" options panel tab, set the "RegExp" algorithm, an enter pattern "A {4,6}

   " (withouts quotes).
   Expected state: all subsequences that consists from "A" with length from 4 to 6 are found.
   Current state: you can't enter the pattern: widget doesn't allow to enter digits.