Search and replace with regular expressions in .net

Regular expressions in .net provides a very easy and convienet way to do search and replace strings using patterns. Here is an example of how this works.

Let's take this string

"This is a good place to do a test. And today is a sunday. A sunny sunday is a great day. A dog has eaten a chicken."

We want to replace every word that comes before the letter 'a' with "xyz". In this case the string should become

"This xyz a good place to xyz a test. And today xyz a xyz A sunny sunday xyz a great xyz A dog has xyz a chicken."

What we want to do is look for a pattern like this -

(anyword)(space)a(space)

Then we want to replace (anyword) with xyz in every occurence in the string. Let's now translate this pattern to a .net regular expression.

(anyword): A bunch of non white space characters together can be denoted by \S+
(space): One more spaces can be denoted by \s+
a: a
(space): One more spaces can be denoted by \s+

So, the complete regular expression becomes \S+\s+a\s+

Now let's see the code

        string strTemp = "This is a good place to do a test. And today is a sunday. A sunny sunday is a great day. A dog has eaten a chicken.";
        string strPattern = @"\S+\s+a\s+";
        string strReplacement = "xyz a ";
        Regex rgx1 = new Regex(strPattern, RegexOptions.IgnoreCase);
        strTemp = rgx1.Replace(strTemp, strReplacement);
        Console.WriteLine(strTemp);

Line 1: The test string
Line 2: We define the regular expression pattern
Line 3: We define the string replacement
Line 4: Create a new Regex object using the regular expression defined above. The second parameter in the constructor - RegexOptions.IgnoreCase means that the search will be case insensitive.
Line 5: We call the Replace() method to do the actual search and replace.

Here is the output: Output