C# Regex Optimization

Question

I have a C# application, which I'm using RegEx to run an expect from a Unix response. I currently have this.

//will pick up :
//  What is your name?:
//  [root@localhost ~]#
//  [root@localhost ~]$
//  Do you want to continue [y/N]
//  Do you want to continue [Y/n]
const string Command_Prompt_Only = @"[$#]|\[.*@(.*?)\][$%#]";
const string Command_Question_Only = @".*\?:|.*\[y/N\]/g";
const string Command_Prompt_Question = Command_Question_Only + "|" + Command_Prompt_Only;

This works as I've tested it with www.regexpal.com, but I think I need some optimization as there are times, it seems to slow way down when I use Command_Prompt_Question.

var promptRegex = new Regex(Command_Prompt_Question);
var output = _shellStream.Expect(promptRegex, timeOut);

I might want to mention I'm using SSH.NET to talk to these Linux servers, but I don't think it's a SSH.NET issue because when I use Command_Prompt_Only it's fast.

Does anyone see any issues with the const string I'm using? Is there a better way to do it?

My project is open source if you feel like you want to go play with it.
https://github.com/gavin1970/Linux-Commander

Code in question: https://github.com/gavin1970/Linux-Commander/blob/master/Linux-Commander/common/Ssh.cs

It's call Linux Commander and I'm attempting to build a virtual linux console with Ansible support.

Have you tried using a cached (static readonly Regex) with RegexOptions.Compiled? I note that your regex is using ECMAScript syntax /g which is not supported by .NET - you also aren't putting each sub-expression in a non-capturing group - is that intentional? — Dai
– Dai, Commented Sep 22, 2020 at 22:16
Wasn't aware that /g wasn't supported. Thanks for that info. So, are you saying by grouping it will make it faster? ([$#])|([.*@(.*?)][$%#])|(.*\?:)|(.*[y/N])|(.*[Y/n]) — user1853517
– user1853517, Commented Sep 22, 2020 at 23:15
btw, static readonly should be used for public, where const is more for private use and would be faster for the reads. This variable isn't something that should change, once I get it set correctly. — user1853517
– user1853517, Commented Sep 22, 2020 at 23:19
That is incorrect: you cannot use const with reference-types like Regex, you can only use static readonly. Saying "const is faster" is an oversimplification: const and static readonly have different semantics and the C# compiler will inline const values even across assembly boundaries (which can be a source of bugs if you don't rebuild when updating assembly dependencies). — Dai
– Dai, Commented Sep 22, 2020 at 23:19
Using /g in ECMAScript regex is the same as enumerating all Match values in the MatchCollection - however your code doesn't seem to evaluate the regex by itself, instead it's done by the .Expect method. Where is that defined and what does it do? — Dai
– Dai, Commented Sep 22, 2020 at 23:20

ΩmegaMan · Accepted Answer · 2020-09-22 23:37:18Z

1

Does anyone see any issues with the const string I'm using?

Yes too much backtracking in those patterns.

If one knows that there is at least one item, specifying a * (zero or more) can cause the parser to look over many zero type assertions. Its better to prefer the+ (one or more) multiplier which can shave a lot of time off of researching dead ends in backtracking.

This is interesting \[.*@(.*?)\] why not use the negative set ([^ ]) pattern instead such as this change:

\[[^@]+@[^\]+\]

Which says anchor off of a literal "[" and the find 1 or more items that are not a literal "@" ([^@]+) and then find 1 or more items that are not a literal "]" by [^\]+.

answered Sep 22, 2020 at 23:37

ΩmegaMan

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2 Comments

user1853517 Over a year ago

I have in the post above, what I'm attempting to capture. There are many scenarios and I must pass them all in with one call to SSH.NET. @ is part of the string I'm looking for. [root@localhost ~]# is just one scenario.

user1853517 Over a year ago

I have taking your suggestion about * vs +.

Dai · Accepted Answer · 2020-09-23 01:07:46Z

-1

Try this:

class Foo
{
    const string Command_Prompt_Only     = @"[$#]|\[.*@(.*?)\][$%#]";
    const string Command_Question_Only   = @".*\?:|.*\[y/N\]";

    const string Command_Prompt_Question = "(?:" + Command_Question_Only + ")|(?:" + Command_Prompt_Only + ")";

    private static readonly Regex _promptRegex = new Regex( Command_Prompt_Question, RegexOptions.Compiled );

    public void Foo()
    {
        // ...

        var output = _shellStream.Expect( _promptRegex, timeOut );
    }
}

edited Sep 23, 2020 at 1:07

answered Sep 22, 2020 at 23:22

Dai

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3 Comments

Poul Bak Over a year ago

/g has no meaning in .net, it matches a literal slash followed by 'g'.

Dai Over a year ago

@PoulBak I left that in my answer because at the time I didn't know if the OP wanted it there or not.

Poul Bak Over a year ago

You should also use non-greedy quantifiers, that will speed up.

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