Unsupported PCRE constructs

\0dd \ddd \0{dd...} octal numbers
\Q...\E quoting
\cx control characters
\C data unit
\h \H horizontal character classes
\v \V vertical character classes
\p{xx} \P{xx} character properties
\X unicode grapheme cluster
boundaries other than ^$
atomic groups
comments
options/modes
subroutines
conditional patterns
callouts
match point reset \K

Other unsupported "things"

[[.hyphen.]] named characters
[[=M=]] whatever this is