Lior.Net

In the following post I will explain what is regular expression, what we can do with it and the special characters that we can use in order to find the specific pattern.

The regular expression provide means to identify strings of text of interest. In other word the regular expression let us search a specific pattern in a text

What we can do with Regular Expression ?

1. Search easily specific pattern

2. Validate a string for example : credit card number, Id number and ect..

3. Find the specific pattern and do some action like replace characters

Example of regular expression in C#:

Private void regExample()

{       string str = “Hello. My name is Inigo Montoya. You killed my father prepare to die.”;

string exp = @”\binigo\b”;

Regex ex1 = new Regex(exp,RegexOptions.IgnoreCase);

// /b in the end and in the begining means that the word is Inigo and not part as other words like Inigojbkbj

// Moreover the option IgnoreCase mean that there is no meaning to the letters case.

Console.WriteLine(ex1.IsMatch(str));

// Match can give you more data where this string is located

Match match = ex1.Match(str);

Console.WriteLine(“Found string ‘” + match.Captures[0].Value + “‘ in:” + match.Captures[0].Index);

Console.WriteLine(ex1.Replace(str, “David”));

// Let’s find Inigo and afterward somewhere in the string should appear the word father

Regex ex2 = new Regex(@”\bInigo\b.*\bfather\b”);

match = ex2.Match(str);

foreach (Capture c in match.Captures)

{

  Console.WriteLine(“Found the pattern in:” + c.Value + ” in Index: “ + c.Index);

}

}

Special Characters that can help us search a pattern:

 

Character	Description	Example
/b	Matches at the position between a word character	/blior/b mean that the search will look for the word lior
|	Causes the regex engine to match either the part on the left side, or the part on the right side	Abc | def – check for abc or def
.	Match any single character	Ab. à can be ab or abc , and and so on..
^	Matches at the start of the string	^ab – the string need to start with ab
[^]	Accept any character expect the characters in bracket	[^d-f] – expect all charcters d or e or f
$	Verify the end of the string	Def$ - check if the string end with def
*	Repeats the previous item zero or more times	a* - a,aa,aaa,aaaa,aaaaaaa
+	Repeat the previous item one or more times	a+ - aa,aaaa,aaaaaa
?	Zero or one of the preceding item	Dogs? -> dog, dogs
{n}	{n} where n is an integer >= 1	B{2} = bb
{n,m}	{n,m} where n >= 0 and m >= n	B{2,4} = bb,bbb,bbbb
{n,}	Repeat the previous item at least n times	D{2} = dd, dddd and so on.
/d	Any number	0-9
/w	Any number or letter	1,2,3,a,b,d,g and so on.
/s	Match any whitespace character