Inside a parser, how do I "back up" one character? #344

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claunia · 2026-01-29T14:34:19Z

claunia commented

2026-01-29 14:34:19 +00:00

Originally created by @deanebarker on GitHub (Jan 26, 2020).

Assuming I've found my pattern, I also want to include the character before if it's a space. So I want to do something like this:

if(slice.PeekCharExtra(-1).IsSpace())
{
   slice.PrevChar();   // Note: this does not exist
}

I saw the Start property on StringSlice, but I can't figure out what character index I'm on now, so that didn't seem to help.

Originally created by @deanebarker on GitHub (Jan 26, 2020). Assuming I've found my pattern, I also want to include the character before _if it's a space_. So I want to do something like this: ``` if(slice.PeekCharExtra(-1).IsSpace()) { slice.PrevChar(); // Note: this does not exist } ``` I saw the `Start` property on `StringSlice`, but I can't figure out what character index I'm on now, so that didn't seem to help.

claunia added the question label 2026-01-29 14:34:19 +00:00

claunia commented

2026-01-29 14:34:21 +00:00

@xoofx commented on GitHub (Jan 31, 2020):

Assuming I've found my pattern, I also want to include the character before if it's a space. So I want to do something like this:

I don't think we are able to handle this case easily. Once a parser has parsed a slice, it will transform it to its internal representation (and we are not fully 100% roundtrip ready, as we don't store all stuffs that a parser could discard e.g trailing spaces)...
So the easiest solution would be to trigger your parser on space and your characters, and match from that instead.

@xoofx commented on GitHub (Jan 31, 2020): > Assuming I've found my pattern, I also want to include the character before if it's a space. So I want to do something like this: I don't think we are able to handle this case easily. Once a parser has parsed a slice, it will transform it to its internal representation (and we are not fully 100% roundtrip ready, as we don't store all stuffs that a parser could discard e.g trailing spaces)... So the easiest solution would be to trigger your parser on space and your characters, and match from that instead.

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Reference: starred/markdig#344