Why doesn't $ work in regular expression after lookahead?

Question

.NET

Hello,
I can't figure out why $ doesn't work in the following situation.
There is a text

123_456
123_456789

, you need to extract 123 making sure that _456 is present and it's all on one line.
For this I use the following regular expression:

^123(?=_456)$

But it doesn't work with $. Why?

Answer 1

lookahead is not a character set, it is an "anchor" that denotes a position.
So you end up with _456 and $ at the same position, so it doesn't work
Add the end of the line inside your lookahead
^123(?=_456$)

Answer 2

Is your text single line or multiline?
Try it in different modes:
Regular expression options
Multiline (?m) Use multiline mode, where ^ and $ match the beginning and end of a line of text (not the beginning and end of the input line).
Singleline (?s) Use singleline mode, where a dot (.) matches any character (not every character except \n).