Learning regular expression step by step tutorials: Step 3

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If you haven't watched the previous post, then check out the related topics Character Classes or Sets If you want to tell the regex engine to match with one of the several characters, then you can form Character Classes/Sets. To form a character set, we just need to place the characters in square brackets like this : [a-z], this will include all the characters from a to z or like this : [ghi], this will include characters specified in the square bracket(g,h and i) Negated Character Classes If you want to tell the regex  not to match with any of the given range/set of characters, then you can use negated character classes. To form a Negated Character Class you need to add ^(A Caret sign) after opening the square bracket. Some of the examples are: [^ghi] character class will match with any character other than g,h and i. [^a-z] character class will match with any characters other than between a to z.	Related Topics Step 1 Learning regular expressions, Step 2 Learning regular expressions
Short hand Character Classes In previous post we have seen that when you use "backslash" with literals other than special characters because "backslash" in combination with other literals creates a regex token which has a special meaning in itself. For example: when "d " is used in combination with "/", it creates a regex token "/d" which matches all digits from 0-9. The above regex token is an example of Short-hand Character Class. A short hand character class has been developed to include character classes which are used more often. Other Examples are: "\w" stands for word character which can be defined as [a-zA-Z0-9_] "\s" stands for white space character which can be defined as [ \t\r\n](space, tab, line break) Repeating Character Classes  You can repeat a character class using ?,* or + operators. If you use "+" operator with a character class like this: [a-c]+  The above expressions tells the regex engine to match with any characters between a,b,c one or more times. Which means [a-c]+ matches with jukibyha since input(jukibyha) has "b" and "a". However most of the times  you want to check if there are any combinations of "aaa","aba","bbc","baa","cca" etc patterns in the given input. To do that we need to use backreferences to check for repeated matched characters. If you didn't understand what i mean by repeated matched characters, then here is the following explanation: If my regular expression is [0-3] and the input is 568935413, then  our regex engine finds the first match "3" and then stops even though "1" is also a valid match. So now "3" is a Matched character by the regex engine. If you want regex engine to keep searching for matched character "3" then you use backreferences to tell regex engine to save the matched characters in its memory.If you use back references, regex engine will find the first match "3", save it in the memory and it will keep searching for another "3" until end of the given input. So if you want to look for repeated matched characters, you can create a regular expression like this ([0-3])\1+.This will tell the regex engine to look for repeated matched characters for one or more time.     Check for next tutorial on regular expressions Regards Sameer Shaik Free Microsoft MCTS 70-536 Examination Training and Preparation

Learning regular expression step by step tutorials: Step 2

If you haven't watched the previous post, then below is the link:
Step 1 Learning regular expressions

Now we will going little further with regular expressions. In previous tutorial, we have seen that matches were done for very simple expression. In practicality we will be looking for more complex expressions to get the exact match. To provide us with these capabilities, Special Characters/Meta Characters are available.

There are 11 characters with special meanings:
1. The opening square bracket [
2. The backslash \
3. The caret ^
4. The dollar sign $
5. The period or dot .
6. The vertical bar or pipe symbol |
7. The question mark ?
8. The asterisk or star *
9. The plus sign +
10. The opening round bracket (
11. The closing round bracket ).

Since the above characters have a special meaning, we cannot find a match  for "2+2" in a string "2+2 equals 4". When you use these special characters for ex: plus sign in  "2+2" it will match with 22,222,2222 but not just 2+2,2,22+2 etc. '+' matches the preceding character one or more times.

If you would like special characters to be treated like literals in the previous example. That is you want "2+2" regex to match with "2+2 equals 4". Then you need to escape these special characters with "backslash(\)" like this: 2\+2 will ignore the "+" sign as literal which will now match with the given string.

Point to remember:  When you use "backslash" with literals other than special characters because "backslash" in combination with other literals creates a regex token which has a special meaning in itself.
For example:
when "d " is used in combination with "/", it creates a regex token "/d" which matches all digits from 0-9.

The following example is simple demonstration of what we've learnt,
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace RegexStep2
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "2+2 equals 4";
            string regex = @"2+2";
            if (Regex.IsMatch(s, regex))
            {
                Console.WriteLine("Given string " + s + " matches with regular expression " + regex);
                Console.ReadKey();
            }
            else
            {
                Console.WriteLine("Given string " + s + " does not match with regular expression " + regex);
                Console.ReadKey();

            }

        }
    }
}

Non Printable Characters:

We can also use special characters to put non printable characters in our regular expressions.Some examples of non printable characters are as follows:

1. "\t" to match a TAB character

2. "\n" to match a Line Feed.

3. "\r" to match a Carriage Return.

etc etc

 

Regards
Sameer Shaik