I need to extract substring from string but starting with the first letter

**example** : 

```
string s1 = "12 x 13 ABC 12@ 15.8" substring =  ABC 12@ 15.8
string s2 = "25 x 32 FER @23.8" substring =  FER @23.8

```

I tried the index of for the letter A or F but it didn't work

thanks