How to extract number which is of variable length from a string using Substring?

I need to extract number from a string column.The length of number in string value is not fixed but it is ended with a special symbol underscore(_).I have tried with Substring but unsuccessful.Please suggest,below sample data for the column

Example String :

FilePath
1002001_Inv_QCR.tiff
100101_Inv_MAN.J开发者_运维问答pg



SELECT SUBSTRING(Filepath,1,6)  from Tblfileinfo

I guess, the following could help:

SELECT
    SUBSTRING(Filepath, 1, CHARINDEX('_', Filepath) - 1)
FROM
   Tblfileinfo

DECLARE @Value NVarChar(255)
SET @Value = 'Something_1002001_Inv_QCR.tiff'
SELECT SubString(@Value, PatIndex('%[0-9]%', @Value), CharIndex('_', @Value, PatIndex('%[0-9]%', @Value)) - PatIndex('%[0-9]%', @Value))

(note: this would also handle the case where the number is NOT at the start of the name)

Specific to the provided table:

SELECT SubString(Filepath, PatIndex('%[0-9]%', Filepath), CharIndex('_', Filepath, PatIndex('%[0-9]%', Filepath)) - PatIndex('%[0-9]%', Filepath))
FROM Tblfileinfo