Ridiculously simple regex that refuses to work

[dis astranagant]

I have a folder full of 1.jpg 2.jpg ... 40.jpg that need leading zeros on the single digit names. I've tried using (\b\d\.jpg) replacing with 0\1 but it's giving me some strange results. Instead of getting 01.jpg out of it I'm getting 01.jpg.jpg.

[truth]

Sounds like you've got some settings to verify:
Options/Ignore/FileExtension=Checked, 1Regex IncludeExt=UNchecked, 11Extensions=Same
Also, since you spec'd \b word-separator, I assumed that filenames vary more than those listed

Since you spec'd .extension in match, I'll stay with that format
To rename .jpg's ending either as: #.jpg (even if nothing precedes it) or WordSeparator#.jpg
So that:
1.jpg -----------> 01.jpg
11.jpg ---------> (not matched for renaming)
Name 1.jpg ---> Name 01.jpg
Name-1.jpg ---> Name-01.jpg
Name 11.jpg --> (not matched for renaming)

1Regex
^(.*[^0-9,\w])*(\d.jpg)$
\10\2