CODEWITHSUNDEEP
c#regexmatchconditional-statements
Kris Verheire
Kris Verheire

Reputation: 133

Regular expressions : How to make a conditional regex

I have the following conditions for my regex : When the string is not empty it should contain the word "internal"

So in other words :

"<link linktype='internal' id='{F88AE8AE-69C4-4E31-95BF-73B110FEE63A}' />" --> OK
"<link linktype='external' id='{F88AE8AE-69C4-4E31-95BF-73B110FEE63A}' />" --> NOK
"test" --> NOK
"" --> OK

I know that an empty string can be checked with : ^$
Or a non empty string with : ^\s*\S
and my internal check simply as : linktype=\'internal\' (for example)

Bringing them together is the hard part. I've been stuck on this, but it doesn't do as expected :

(?(?=^\s*\S)linktype=\"internal\"|^$)

Can anyone help ?

Upvotes: 3

Views: 1869

Answers (8)

Petr Behensk&#253;
Petr Behensk&#253;

Reputation: 620

string regex = "^(?:\\n|.*linktype=([\'\"])internal\\1.*\\n)";
var options = RegexOptions.Multiline);
var reg = new Regex(regex, options);

Upvotes: 0

Kevin Brock
Kevin Brock

Reputation: 8944

Perhaps I should add my own answer. The answers so far have used capturing groups which are slightly more costly. To use an "or" condition with a non-capturing group:

(?:^$)|(?:linktype=['\"]internal['\"])

There is no need for anchors on the second part as an RE by definition will match anywhere within the string without the anchors.

Also, to use an "and" condition in a RE you simply concatenate the rules together. This is how the above RE is formed actually. It is (anchor start AND anchor end) OR (an l AND i AND n ... AND character set ['"] AND i AND ... etc...)

Upvotes: 0

codefly
codefly

Reputation: 640

something like this should do it:

^.*linktype='(internal|)'.*$

Upvotes: 0

Giuseppe Romagnuolo
Giuseppe Romagnuolo

Reputation: 3402

"<link linktype='internal' id='{F88AE8AE-69C4-4E31-95BF-73B110FEE63A}' />" --> OK
"<link linktype='external' id='{F88AE8AE-69C4-4E31-95BF-73B110FEE63A}' />" --> NOK
"test" --> NOK
"" -->

If linktype='internal' is matched you don't really care of what is before and after the linktype='internal', you will anyway get a match:

(^$)|(linktype='internal')

Upvotes: 0

Tomas Jansson
Tomas Jansson

Reputation: 23472

what about something like

(^$)|(^.*linktype=\"internal\".*$)

--Tomas

Upvotes: 0

Antoine Aubry
Antoine Aubry

Reputation: 12469

In this particular case, you can use:

^(.*linktype=['"]internal['"].*)?$

Otherwise, it is easier to write a regex for each case separately and then enclose them in parenthesis and use a 'or' to include them in a single expression:

(^$)|(^.*linktype=['"]internal['"].*$)

This will match either ^$ or ^.*linktype=['"]internal['"].*$.

Upvotes: 1

dlev
dlev

Reputation: 48596

You could try (^$)|(^.*linktype=\"internal\".*$)

Either the empty string, or a string with the text linktype="internal".

Upvotes: 1

dee-see
dee-see

Reputation: 24078

Since you mentioned C#, you might as well try this:

if(str.Length == 0 || str.Contains("internal"))

It works and it's simple.

Upvotes: 2

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