if your tool/language support look-behind, this works: match the first _m
till EOL
. also ignore the leading _
(?<=_)m.*
test with grep:
kent$ echo "Xxx x xx:xx:xx xmmxxx XXXXXX: XXXXXXX:XXX: xxx_Mxxx_Xxxxxx_mxxxxxmmxx [XXX xxxx."|grep -Po '(?<=_)m.*'
mxxxxxmmxx [XXX xxxx.