Remove all letters from specific word except the first

I have a load of files named

Firstname Secondname_001
Firstname Secondname_002

and i want them to be

Firstname S_001

how can i specify to remove everything either 2 characters after the space or from the second character of word 2

Try Remove pattern...

Pattern: [^ ]* \w\K[^_]*
Use regular expressions

Edit....

More simply...

Pattern " \w\K[^_]*"
(note: pattern starts with space)

Reply to #2:
oh i see... you need to include the caret to signify the character.....

lol, i was so close.

and what does the K do?

Reply to #3:

"^" means "NOT"
So [^xyz] matches any character not listed (ie NOT x,y or z)

The metacharacter \K is an instruction not to include the preceding matched characters in the preceding match.

Thus the regex "[^ ]* \w\K" must match a string of any number of non-space characters followed by a space and a single word character but the matched characters are not included in the final text to be removed.

The remainder of the regex, "[^_]*", then matches any characters until an underscore is encountered - and this string is removed.