/
 ^           # match beginning of string
 [a-z]       # match a letter for the first char
 (?:         # start non-capture group
   _?/i        # match 0 or 1 '_'
   [a-z0-9]+   # match a letter or number, 1 or more times
 )*          # end non-capture group, match whole group 0 or more times
 $           # match end of string
 .           # match any 1 char
 [^rtv]      # match everything except this group of chars
 +           # one or more repetitions of a char/group (eg «is+»)
 A|B         # match A or B
 {}          # number of repetitions («n,» - n or more)(«n,m» - [n,m]) 
/