ID 226847 - The File Browser cannot collapse sequences with a custom file extension if the extension starts with a number

Problem summary
The File Browser cannot collapse sequences with a custom file extension if the extension starts with a number.

Customer reported version
nuke.10.0v1

Customer reported platform
windows10

Steps to reproduce

1) Have two file sequences with custom extensions, one that starts with a number (e.g. ".5ths") and one that starts with a letter (e.g. ".ths"). You can test this by renaming another file type, e.g. .dpx files.
2) Try to read the files into Nuke, when you view the files in the File Browser and check the "sequences" check-box Nuke will still show each file as they are custom extensions.
3) Use the nuke.addSequenceFileExtension() function to add the custom extensions to the list of extensions Nuke treats as sequences in the file browser by running the following commands in the Script Editor

nuke.addSequenceFileExtension('5ths')

nuke.addSequenceFileExtension('ths') 

4) Attempt to read the files in again and note that the "ths" files are now collapsed as a sequence but the "5ths" files are not.

Reproduced by support
This problem has been reproduced in:

Nuke 10.0v3 - Windows 7, OSX 10.11, CentOS 6.7

Nuke 9.0v9 - Window 7

Nuke 9.0v1 - Windows 7

Nuke 8.0v1 - Window 7

Nuke 7.0v1 - Windows 7

Expected behaviour
After running the nuke.addSequenceFileExtension() command the File Browser should collapse file sequences that start with numbers.

Actual behaviour
Nuke does not collapse the sequences for files with extensions starting with a number.