extracting social security number from an xml file using grep and cut

file.txt

<field10>123-45-6789</field10>

<field10>123-45-6789</field10>

<field10>123-4-6789-</field10>

here is the regexp

([0-9]+-)+[0-9]+<

and the command is 

grep -o '\([0-9]\+-\)\+[0-9]\+<' file.txt | cut -d "<" -f 1