General shell syntax, using unix pattern matching of the case statement to check the filename format, without separating the filename in different parts:

Code:

case $inputFile in
  (*[![:alnum:]]*_*_*.*|*_*[![:digit:]]*_*.*|*_*_*[![:digit:]]*.*|*_*_*.*[![:alnum:]]*)
    echo "filename is wrong"
  ;;
  (*_*_*.*)
    echo "filename is correct"
  ;;
  (*)
    echo "filename is wrong"
  ;;
esac

--
Or bash/ksh93/zsh, same thing, but this time using extended regular expression matching:

Code:

if [[ $inputFile =~ ^[[:alnum:]]+_[[:digit:]]+_[[:digit:]]+\.[[:alnum:]]+$ ]]; then
  echo "filename is correct"
else
  echo "filename is wrong"
fi

--
A quick general (POSIX) way to split the filename in different variables, using a here-document:

Code:

IFS='_.' read name major minor ext << EOF
$inputFile
EOF

Or bash/ksh93/zsh, using a here-string:

Code:

IFS='_.' read name major minor ext <<< "$inputFile"

---
Note: this will globally change IFS and not local to the array assignment. Also the order in which these two assignments will be executed is not defined, so it is best to use a semicolon or newline to separate them and to make sure the IFS assignment is done before the array assignment and save IFS and restore it later,

Code:

oldIFS=$IFS
IFS="_."
ARR=( $inputFile )
IFS=$oldIFS

or use (bash):

Code:

IFS="_." read -a ARR <<< "$inputFile"

In the latter case IFS does get set local to the read command.