Java (JDK), 142(削除) 151 (削除ここまで)(削除) 142 (削除ここまで) 135 bytes
s->{var m=new java.util.TreeMap();for(var c:s.toLowerCase().replaceAll("[^a-z]"zA-Z]","").toCharArray())m.merge(cc|=32,1,(a,b)->(int)a+(int)b);return m;}
Saved 915 bytes thanks to Kevin Cruijssen.
Java (JDK), 142 bytes
s->{var m=new java.util.TreeMap();for(var c:s.toLowerCase().replaceAll("[^a-z]","").toCharArray())m.merge(c,1,(a,b)->(int)a+(int)b);return m;}
Saved 9 bytes thanks to Kevin Cruijssen.
Java (JDK), (削除) 151 (削除ここまで)(削除) 142 (削除ここまで) 135 bytes
s->{var m=new java.util.TreeMap();for(var c:s.replaceAll("[^a-zA-Z]","").toCharArray())m.merge(c|=32,1,(a,b)->(int)a+(int)b);return m;}
Saved 15 bytes thanks to Kevin Cruijssen.
Java (JDK), 151142 bytes
s->{var m=new java.util.TreeMap<Character,Long>TreeMap();s;for(var c:s.toLowerCase().replaceAll("[^a-z]","").charstoCharArray().forEach(c->m)m.merge((char)c,1L1,(a,b)->a+b>(int)a+(int)b);return m;}
Saved 9 bytes thanks to Kevin Cruijssen .
Java (JDK), 151 bytes
s->{var m=new java.util.TreeMap<Character,Long>();s.toLowerCase().replaceAll("[^a-z]","").chars().forEach(c->m.merge((char)c,1L,(a,b)->a+b));return m;}
Java (JDK), 142 bytes
s->{var m=new java.util.TreeMap();for(var c:s.toLowerCase().replaceAll("[^a-z]","").toCharArray())m.merge(c,1,(a,b)->(int)a+(int)b);return m;}
Saved 9 bytes thanks to Kevin Cruijssen .
Java (JDK), 151 bytes
s->{var m=new java.util.TreeMap<Character,Long>();s.toLowerCase().replaceAll("[^a-z]","").chars().forEach(c->m.merge((char)c,1L,(a,b)->a+b));return m;}