Regula expressions Add _ after numbers

[jme]

Hi,

I have to rename files names starting with numbers ( between 8 to 9 numbers) followed by characters and sometimes any others numbers.

What can be the regular expression to add a _ just after the first serie of numbers ?

Names always start with numbers.

example 123456blabla.ext
123456_blabla.ext

[antp]

Try this expression:
(\d+)(.+)
with this name mask:
$1_$2