r - Find pattern containing string not preceeded by other string -

i use grepl function in r find if string contains something, on condition not preceeded else.

so example wanted find string includes pattern 'xx', long not preceeded 'yy'. so:

'123xx45' world return true

'123yy4xx5' return true 'yy' not preceding 'xx'

however '123yyxx45' return false.

please let me know if unclear or better example.

how grepl('(?<!yy)xx', c('123yy4xx5','123xx45','123yyxx45'), perl=true)?