How to consume head of a string?

[oovm]

I'm writing a lexer that needs to consume the head of a string based on a pattern.

For example, for [1-9]+

input	match	offset
123abc	123	3
0123abc	×

Which method on Regex should I use to reject the second input?

[BurntSushi]

Change your regex to ^[1-9]+?

[oovm]

Yes, this works, but I can't simply format as^({}) in preprocessing, some rules may use (?mode) or something.

[BurntSushi]

I don't see the problem. Can you please provide an example that doesn't work?

[oovm]

You are right, I found that what I encountered was due to double escaping, thank you