unix - Can't extract pattern from filename -

getting errors following sed command:

echo 20130521_onepki_p107336_app.pfx | sed -e 's/_\([pp][0-9]+\)_/\1/' 

instead of returning p107336, returning full filenam 20130521_onepki_p107336_app.pfx.

any ideas why happening, , how can restrict output pattern like?

the captures should escaped parentheses , can use case-insensitive match i, also, replacing capture part captured part no changes made. 1 matches entire line , replaces captured pattern:

sed -e 's/.*_\([pp][0-9][0-9]*\)_.*/\1/'