cant get Regex to work (.NET) without a space in text

im trying to split a series of lines based on the LAST underscore character.

its something to do with that one line not having a space in the text & im not familiar with regex concepts like lookahead and lookbehind

using

^(.*?)_(\w+?)*$

run against

2022-366_DA00_Cover Sheet_C
2022-366_DA01_Locality Plan_E
2022-366_DA02_Site Plan_H
2022-366_DA03_Delivery Plan_E
2022-366_DA04_Floorplan_D
2022-366_DA05_Roof Plan_D
2022-366_DA06_Front  Side Building Elevations_F
2022-366_DA07_Drivethru  Rear Building Elevations_D
2022-366_DA08_External Finishes Schedule_A

produces

2022-366_DA00_Cover Sheet==C
2022-366_DA01_Locality Plan==E
2022-366_DA02_Site Plan==H
2022-366_DA03_Delivery Plan==E
**2022-366==D**
2022-366_DA05_Roof Plan==D
2022-366_DA06_Front  Side Building Elevations==F
2022-366_DA07_Drivethru  Rear Building Elevations==D
2022-366_DA08_External Finishes Schedule==A

You do not have to repeat the capture group as you want to match 1 or more word characters (also if you use * then the whole group could also be optional)

Matching the word characters \w+? and the .*? do not have to be non greedy.

If you want to match a single uppercase char A-Z you could also use [A-Z] instead of \w+

You might write the pattern excluding matching an underscore from the word characters:

^(.*)_([^\W_]+)$

The pattern matches:

• ^ Start of string
• (.*) Capture group 1, match the whole line
• _ Match _
• ([^\W_]+) Capture group 2, match 1+ word chars except for _
• $ End of string

See a regex demo

that:

^(.?)_(\w+?)$

does not seem right to me, as there is this additional quantifier on the second group? Furthermore I don't think you want the '?' quantifier on either of those groups. Here is what I read from your regex:

^     ... start of the line
(...) ... first group
.?    ... every character matches(.), if there is exactly zero or one of them(?)
_     ... match an underscore(due to default greedy matching, this will match on the last underscore)
(...) ... second group
\w+?  ... any word character will match if there is one or more of them. the question mark is too much, as there can only be one quantifier.
$     ... end of the line

here is what I would have tried using:

^(.*)_(.*)$

which translates to:

^     ... start of the line
(...) ... first group
.*    ... match every character (.) if there is any (*)
_     ... matches the last(remember the greedy thing) underscore
(...) ... second group
.*    ... match every character if there is any
$     ... end of the line

this should bring the needed information stored after the last matching underscore.