Regular expression to match 0 or more occurrences not working

Question

This question may sound stupid. But I have tried several options and none of them worked.

I have the following in a string variable.

string myText="*someText*someAnotherText*";

What I mean by above is that, there can be 0 or more characters before "someText". There can be 0 or more characters after "someText" and before "someAnotherText". Finally, there can be 0 or more occurrences of any character before "someAnotherText".

I tried the following.

string res= Regex.Replace(searchFor.ToLower(), "*", @"\S*");

It didn't work. Then I tried the following.

string res= Regex.Replace(searchFor.ToLower(), "*", @"\*");

Even that didn't work.

Can someone help pls ? Even though I have mentioned "*" to indicate 0 or more occurrences, it says that I haven't mentioned the number of occurrences.

Answer 1

Unlike the DOS wildcard character, the * character in a regular expression means repeat the previous item (character, group, whatever) 0 or more times. In your regular expression the first * has no preceding character, the second one follows the t character, so will repeat that any number of times.

To get '0 or more of any character' you need to use the composition .* where . is 'any character' and * is '0 or more times'.

In other words to search for someText followed any number of characters later by someAnotherText you would use the following Regex:

var re = new Regex(@"someText.*someAnotherText");

Note that unless you specify otherwise by putting start/end specifiers in ( ^ for start of string, $ for end) the Regex will match any substring of the test string.

Tests for the above, all returning true:

re.IsMatch("This is someText, followed by someAnotherText with text after.");
re.IsMatch("someTextsomeAnotherText");
re.IsMatch("start:someTextsomAnotherText:end");

And so on.

In Regex terms * is a quantifier. Other quantifiers are:

?     Match 0 or 1
+     Match 1 or more
{n}   Match 'n' times
{n,}  Match at least 'n' times
{n,m} Match 'n' to 'm' times

All apply to the preceding term in the Regex.

Placing a ? after another quantifier (including ? ) will convert it to lazy form, where it will match as few items as it can. This will allow following terms to also match the terms you specified.

Answer 2

The regular expression to match 0 or more occurrences of any character is

.*

where . matches any single character and * matches zero or more occurrences of it.

(This answer is a quick reference simplification of the current answer.)