Hostthe literal text Host (case sensitive) \s+any whitespace character, repeated at least once
host2the literal text host2 (case sensitive) \sany whitespace character
.*?any character, repeated any number of times
IdentityFilethe literal text IdentityFile (case sensitive) \s+any whitespace character, repeated at least once
Group identity(?<identity>\S+) \S+any non-whitespace character, repeated at least once
s modifier: single line. Allows a dot to match line terminators