Regex that matches sets of numbers at the start of filename : we have filenames that contain product numbers at the start and based on this we apply processing when adding them to the system

i need a regex that should match the following

    70707_70708_70709_display1.jpg
    70707_Front010.jpg

and NOT these 

    626-this files is tagged.jpg
    1000x1000_webbanner2.jpg
    2000 years ago_files.jpg
    626gamingassets_styleguide.jpg

i have a regex that almost does what i want except for one case highlighted below

    \d{3,}(?=_)

 

    70707_70708_70709_display1.jpg - success 3 matches {70707,70708,70709}
    70707_Front010.jpg -             success 1 match {70707 }
    626-this files is tagged.jpg -   success 0 matches
    1000x1000_webbanner2.jpg -       fail  1 match {1000}
    2000 years ago_files.jpg -       success 0 matches
    626gamingassets_styleguide.jpg - success 0 matches

i have a regex test to illustrate this here
https://regex101.com/r/qKkC4m/1

The regex should only look for sets of numbers at the beginning.