bash - search and replace with sed not working -

i use sed search , replace, doesn't seem working. guess i'm making stupid mistake can't spot.

here file.txt:

pfpr03  pa1448770 pfpr03  pa1448780 pfpr03  pa1448790 pfpr03  pa1448800 pfpr03  pa1448810 pfpr03  pa1448830 pfpr03  pa1448840 pfpr03  pa1448850 pfpr03  pa1448860 pfpr03  pa1448870 

i want change pa1448770 pa14_48770, ie. adding underscore after pa14.

here few examples of i've tried, none of them affecting file @ all:

sed 's/^pa14/pa14_/g' file.txt sed 's/^pa14([0-9]*)/pa14_\1/' file.txt  

any appreciated,

it because ^ means line start. use \b word boundary:

sed 's/\bpa14/pa14_/g' file pfpr03  pa14_48770 pfpr03  pa14_48780 pfpr03  pa14_48790 pfpr03  pa14_48800 pfpr03  pa14_48810 pfpr03  pa14_48830 pfpr03  pa14_48840 pfpr03  pa14_48850 pfpr03  pa14_48860 pfpr03  pa14_48870 

ps: on osx (bsd) sed use:

sed 's/[[:<:]]pa14/pa14_/g' file