How to create Rex to extract a value from string?

[Coder1a]

Hello,

I am new to splunk rex, so need help for regex. In logs, i have extracted  string, however again i need to extract a value from string.

Example string :  "Error exception for fetching data =1234567890_1"

Question: From  above string, how can i use rex to get value :1234567890

Request you to please help.

Thanks.

[yuanliu]

See answer in Rex for extracting a value from string. (Alternatively, delete this duplicate question.)