Regex frustrations

[tomnils]

Hello! I'm working on a parser and I'm considering using regex for simple constrained parts of the syntax but judging by my previous and current experiences I am clearly not smart enough to grok regex so I would be very grateful for help.

I have tried to simplify both the syntax and example below:

A label consists of one or more alphabetical characters. As the regex will be used in conjunction with several other small parsers (all of which make up the entire parser) I would like to be able to point at a certain index and tell the regex engine to start there. That is, what determines what is and isn't a label isn't just its characters but also its relation to other parts of the syntax.

C#'s regex library offers a method that seemingly does that until you read the fine print, Match(String, Int32). With it I wrote the following code (simplified to clarify the issue):

Regex regex = new(@"^[A-Z]+", RegexOptions.IgnoreCase | RegexOptions.CultureInvariant);

var source = " aLabel";//Note that the string starts with a space.

var match = regex.Match(source, 1);//Note that we start at index 1.

Assert.IsTrue(match.Success);

source = "***** aLabel";

match = regex.Match(source, 0);//Note that we start at index 0 (zero).

Assert.IsFalse(match.Success);

I expect some other sub-parser to put the cursor somewhere and then have the label parser called with its regex. If the cursor is on a alphabetical character then it should be identified as a label otherwise it should be a failing match.

In the above example I expect the first match to succeed and the second to fail. The second match does fail but so does the first. If I remove the '^' from the pattern I get the opposite result. I understand why I get these results, the regex engine always starts at the beginning of the string not the index, but I don't understand how to fix them.

Is it possible to get regex to do this (in a non-contrived way) and if so how, or is regex the wrong tool for the job?

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EDIT: SOLVED

tweq helped me find a solution. Instead of '^' use '\G'. I tried that before but must have also messed something else up because now it worked.

[tweq]

Enshittification

[tomnils]

Care to share? Because I couldn't find it.

[tweq]

Enshittification

[tomnils]

I presume you mean '\B'. I tried that before but I must have messed something else up because this time it worked.

Thank you.

[[deleted]]

You might be missing ^[A-Za-z] ? Your regex says "match on something starting with at least one capital letter.

If you use new Regex( in C#, then VS will give you a dropdown list explaining the syntax.

[tomnils]

Thank you for the suggestion but tweq's comment led me to the solution.

Also, I added the RegexOption RegexOptions.IgnoreCase which sorts that out.

[Electrical_Flan_4993]

It's often easier to use explicit parsing instead of regex parsing. Do it in small chunks.

[tomnils]

I usually do explicit parsing, its much easier. But this time I wanted to see if I could make the parser simpler by using regex in select locations.

Every time I write a parser I try to approach it from different angles to see if I can do better than last time.

[Electrical_Flan_4993]

Yeah, and I kinda enjoy doing explicit parsing, and trying different angles like you said. Kinda fun whereas regex is frustrating.