Direi che 's' va bene, probabilmente ci sarebbe da capire come e perché 'm' non interpreta i ritorno a capo.
In effetti 's' serve allo scopo:
s (PCRE_DOTALL)
If this modifier is set, a dot metacharacter in the pattern matches all characters, including newlines. Without it, newlines are excluded. This modifier is equivalent to Perl's /s modifier. A negative class such as [^a] always matches a newline character, independent of the setting of this modifier."