Search pattern task can show incorrect progress for regexp algorithm

Scenario to reproduce:

1. Open "data/samples/FASTA/human_T1.fa".
2. Open "Search pattern" options panel tab.
3. Set "regexp" algorithm, enter pattern

   CAcg.{5,200}
   

   Expected state: the search task starts, its progress is adequate.
   Current state: you can see that the task progress sometimes unexpectedly decreases.