1. Improving your code’s readability by documenting regular expressions
2. Creating conditional string replacement by using MatchEvaluators
3. Speeding up regular expressions by compiling them, caching them in memory and pre-compiling them to their own DLL.

If you are new to regular expressions in C# have a look at the theory of regular expression in Regular Expressions : The Basics. The second post Regular Expressions in C#: Practical Usage introduced the most common uses of regular expressions.

Documenting your Regular Expressions

Regular expressions can make for fine alphabet soup. The following expression validates an e-mail address and it does a good job at it. It is also very intimidating at first. So just imagine rereading your code after a few weeks, what is going on in there?

string validEmail = @"\b([a-zA-Z0-9._%+-]+)@([a-zA-Z0-9.-]+\.[a-zA-Z]{2,4})\b";

With a little squinting you see that I would like to extract two groups: the username part, and the domain name part. C# allows us to name each group to make things a little easier to read. We can use the ?<groupname> pattern to name each group.

A little rewrite can make our expression a lot easier to read. C# offers the “#” character to document our expressions in line.

       static string validEmail = @"\b    # Find a word boundary
                       (?<Username>       # Begin group: Username
                       [a-zA-Z0-9._%+-]+  #  Characters allowed in username, 1 or more
                       )                  # End group: Username
                       @                  # The e-mail '@' character
                       (?<Domainname>     # Begin group: Domain name
                       [a-zA-Z0-9.-]+     #  Domain name(s), we include a dot so that
                                          #  mail.dijksterhuis is also possible
                       .[a-zA-Z]{2,4}     #  The top level domain can only be 4 characters
                                          #  So .info works, .telephone doesn't.
                       )                  # End group: Domain name
                       \b                 # Ending on a word boundary
                       ";

Because we have added a lot of spaces and new lines to our expression we need to tell Regex about them by specifying the RegexOptions.Multiline and RegexOptions.IgnorePatternWhitespace options.

          string testEmail = "martijn@dijksterhuis.org";
          Regex TestValidEmail = new Regex(validEmail,RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace);

           // Test the e-mail address
           Match TestResult = TestValidEmail.Match(testEmail);

           if (TestResult.Success)
           {
                Console.WriteLine("E-mail is: {0}@{1}",TestResult.Groups["Username"].Value,
                                                         TestResult.Groups["Domainname"].Value);
           }

Conditional string replacement

The RegEx.Replace method allows you to use substitution parameters to change the original content around. In a previous post we looked at how we could swap two words around by using grouped patterns and the $1 and $2 conditional replacement names.

Regex Replacer = new Regex(@"(\w*) (\w*)");
string Input = "Molly Mallone";
string Output = Replacer.Replace(Input,"$2 $1");
Console.WriteLine(Output);

That is sufficient if you just want to move the data around a little, but it would be nice if you could make a replacement conditional on some external condition. The Regex.Replace method allows you to specify a MatchEvaluator which does just that. MatchEvaluator is a delegate which takes Match as a parameter and returns the replacement string.

Handy for example if you are cleaning up a mailing list and want to conditionally update some, but not all, e-mail addresses. In the following code example we know that mail.dijksterhuis.org is now served by smtp.dijksterhuis.org, so we want to move all those users to the new domain name and leave all other e-mail addresses the same.

using System;
using System.Text.RegularExpressions;

namespace RegularExpression
{
	class MainClass
	{

    	static string validEmail = @"\b   			# Find a word boundary
							  (?<Username>			# Begin group: Username
							  [a-zA-Z0-9._%+-]+     #  Characters allowed in username, 1 or more
							  )                     # End group: Username
							  @					    # The e-mail '@' character
							  (?<Domainname>        # Begin group: Domain name
							  [a-zA-Z0-9.-]+        #  Domain name(s), we include a dot so that
                                                    #  mail.dijksterhuis is also possible
							  .[a-zA-Z]{2,4}        #  The top level domain can only be 4 characters
													#  So .info works, .telephone doesn't.
							  )                     # End group: Domain name
                              \b
							  ";

		public static string UpdateDomainNames(Match match)
		{
			if (match.Groups["Domainname"].Value=="mail.dijksterhuis.org")
			 return match.Groups["Username"].Value + "@" + "smtp.dijksterhuis.org";
			return match.Groups[0].Value; // The original
		}

		public static void Main(string[] args)
		{

		   Regex TestValidEmail = new Regex(validEmail,RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace);

		   string[] MailingList = new string[] { "martijn@dijksterhuis.org",
												 "user@mail.dijksterhuis.org",
												 "willy@wortel.org"};

		   foreach(string email in MailingList)
		   {
				// Conditionaly replace e-mail addresses
				Console.WriteLine( TestValidEmail.Replace(email,UpdateDomainNames) );
		   }

		}
	}
}

Speeding up regular expressions by compiling them

Regular expressions can be quite slow and in another post I found that a simple string replacement routine was some 40 times faster than the equivalent regular expression. Often you will want to stick with the regular expression as it will save you many lines of coding.

As the RegEx class encounters your expressions it compiles them to an internal format. It steps through this internal format each time you query the expression. It is also possible compile your expression to MSIL (the byte code to which C# is compiled) directly. In the best possible scenario the Just-In-Time compiler then translates this MSIL code directly to machine code giving another speed boost to your expression.

A note of caution: According to the MSDN team the increase in speed can be up to 30% which is nice but certainly isn’t amazing.

You can do this by setting the RegexOptions.Compiled option when you create a new RegEx:

Regex theExpression = new Regex(thePattern,RegexOptions.Compiled);

The penalty for this is the time to compile the expression which can add significantly to your applications start-up time. So although “compiled” might sound faster it might actually be slower. This is best applied if you frequently use the expression and it has a very long lifetime.

The expression cache

If you use many regular expressions the RegEx cache is also an important factor in how quickly your code executes. Each time you define a regular expression the library needs to parse it. If you frequently use a small set of regular expressions they won’t be compiled over and over again, instead they come from a cache. You will find that .NET/C# caches the last 15 expressions. Any more and it will have to recompile them as it encounters them.

It is possible to expand the size of the cache by setting the Regex.CacheSize property to a higher value. This is probably best done after you made an overview of how many expressions are used by your code.

Compiling to an assembly

For compiling a regular expression to MSIL you need to pay a hefty price. But with your project about to ship it might be worthwhile to investigate taking your most frequently used regular expressions and putting them pre-compiled into a new assembly. The Regex.CompileToAssembly method performs this function. You will have to write a separate program to do the actual compilation, but once done you can link in the regular expression like any other assembly to your main application.

You can use the following class to create your own set of regular expressions and save them to a new assembly:

using System;
using System.Collections;
using System.Text.RegularExpressions;

namespace CompileExpression
{
	class MainClass
	{
		// Add the expressions to the hash table
	 	public static Hashtable TheExpressions = new Hashtable();

		// CompileExpressions
		public static void CompileExpressions(string AssemblyName)
		{
			// Reserve space for each expression
			RegexCompilationInfo[] CI = new RegexCompilationInfo[TheExpressions.Count];

			int Cnt = 0;
        	foreach(DictionaryEntry de in TheExpressions)
        	{
				CI[Cnt++] = new RegexCompilationInfo((string)de.Value,		  // the reg. ex pattern
				                                     RegexOptions.Compiled,   // Options to specify
				                                     (string)de.Key,		  // name of the pattern
				                                     "TheRegularExpressions", // name space name
				                                     true );                  // Public?
        	}

		   // Create a new assembly name structure
		   System.Reflection.AssemblyName aName = new System.Reflection.AssemblyName( );

		   // Assign the name
  		   aName.Name = AssemblyName;

		   // Compile all the regular expressions into the assembly
  		   Regex.CompileToAssembly(CI, aName);
		}

		public static void Main(string[] args)
		{
			// Add two expressions to the collection
			TheExpressions.Add("FindHTML",@"(<\/?[^>]+>)");
			TheExpressions.Add("FindTCPIP", @"(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})");

			// Compile them to my new assembly called "RegEx"
			CompileExpressions("RegEx");
		}
	}
}

This will create a file called “RegEx.dll” in the home directory of your program. The next step is to verify if this works as advertised. Create a new project in Visual Studio and add a reference (in the Solution Explorer right click the name of the new project and click “Add Reference…” and navigate to where the RegEx.DLL file is located.

The following class will load the FindTCPIP expression from the DLL and execute it:

using System;

namespace TCPSolution
{
   class Program
   {
       static void Main(string[] args)
       {
           TheRegularExpressions.FindTCPIP MatchTCP = new TheRegularExpressions.FindTCPIP();

           if (MatchTCP.Match("10.0.0.6").Success)
           {
               Console.WriteLine("This works!");
           }
       }
   }
}

Regular Expressions and Mono

I tested, prodded and played with the code for these regular expression posts on MonoDevelop and Mono. With the exception of the final “Compile to DLL” example. The code for that example compiles but on execution it will throw an “Not Implemented” exception in Regex.CompileToAssembly.

The end

This ends the mini series of three posts on regular expressions. I hope you have enjoyed them. The previous posts in this series are:

• Regular Expressions : The Basics. The theory behind regular expressions.
• Regular Expressions in C#: Practical Usage Examples of common usage.

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This is a post from Martijn's C# Coding Blog.

This is the second post in the C# regular expression series and it follows up on “Regular Expressions in C# – The Basics” which explained the theory behind Regular expressions in C#. In this post we look at how to make practical use of regular expressions in our C# code.

This post touches on four major regular expression subjects:

• String Comparison – does a string contain a particular sub-string?
• Splitting a string into segments – we will take an IPv4 address and retrieve its dotted components
• Replacement – modifying an input string
• Stricter input validation – how to harden your expressions

String Comparison – finding valid HTML tags

One of the essential functions of expressions are their ability to find if a string is contained inside another one. The RegEx.Matches method tests if a given string matches the pattern.

We start with a simple example: finding out where the letter “a” is mentioned in a sentence:

            string Input = "apples make for great party accessories";
            Regex FindA = new Regex("a");

            foreach(Match Tag in FindA.Matches(Input))
            {
                Console.WriteLine("Found 'a' at {0}",Tag.Index);
            }

That was almost too easy. Regular expressions really shine if you don’t know exactly what you are looking for but you can describe it. In the following example we will look for all valid HTML tags in an input string.

What is a valid HTML tag? <code>, </code>, <b>,<img src=”">, </br> are all valid HTML tags.

Regex HTMLTag = new Regex(@”(<\/?[^>]+>)”);

To break this down:

1. All valid HTML tags start with a “<”
2. They might or not have a forward slash (we need to escape the forward slash) \/?
3. There is at least one or more characters which are not “>”
4. The tag ends with a “>”

The following code example searches for all valid HTML tags in the input string:

using System;
using System.Text.RegularExpressions;

namespace RegularExpression
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            Regex HTMLTag = new Regex(@"(<\/?[^>]+>)");

            string Input = "<b><i><a href='http://apple.com'>Ipod News</a></b></i>";

            foreach(Match Tag in HTMLTag.Matches(Input))
            {
                Console.WriteLine("Found {0}",Tag.Value);
            }
        }
    }
}

Resulting in:

Splitting a string into parts

Parentheses () not only allow you to group your expressions into parts they allow you to split a single string into multiple segments which we can inspect individually. To demonstrate we will use a regular expression to split an IPv4 address into its components.

A decimal TCP/IP address looks like XXX.XXX.XXX.XXX with X being a decimal number. Each column has at least 1 digit, and a maximum of 3. So a single column can be described as “(\d{1-3})“. There are four columns, each seperated by a dot. The dot (.) has a special meaning in regex so we need to escape it. (\.)

The Regex.Match method returns a new Match instance. We can now test Match.Success to see if the input string matched the TCP/IP address pattern. Through the Match.Groups property can we then extract each of the four IP address columns.The zero entry in the Groups property is alway the complete match, in this case “10.0.0.6″. The [1] entry contains the first groups contents, [2] the second etc.

            string IPMatchExp = @"(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})";
            Match theMatch  = Regex.Match("10.0.0.6",IPMatchExp);
            if (theMatch.Success)
            {
                Console.WriteLine("{0}.{1}.{2}.{3}",theMatch.Groups[1].Value,
                                                      theMatch.Groups[2].Value,
                                                      theMatch.Groups[3].Value,
                                                      theMatch.Groups[4].Value);
            }

String Replacement

Often is useful to manipulate a string, by replacing the matched pattern with something new. The RegEx.Replace method allows us to specify a pattern to look for and a replacement string.

The following example matches the last character and space following each word and replaces it with “b_”.

            Regex Replacer = new Regex(@"\w "); // Single [a-zA-Z] followed by a space
            string Input  = "ax bx sax dam pom";
            string Output = Replacer.Replace(Input,"b_"); // Replace all items found with a b and underscore
            Console.WriteLine(Output);

Substitution Patterns

What to do if you would like to flip parts of a string? C# offers several substitution patterns for this. Substitution patterns can only be used in a replacement string, and are used in combination with grouping.

They are useful if you would like to format the results of the match. A common task is to flip two words around. In the below example we flip the name “Molly Malone” into “Malone Molly”:

            Regex Replacer = new Regex(@"(\w*) (\w*)");
            string Input  = "Molly Mallone";
            string Output = Replacer.Replace(Input,"$2 $1");
            Console.WriteLine(Output);

The regular expression is defined as two groups of words (\w*) separated by a space. Each group can be referred to with a substitution pattern. $1 refers to the first group, $2 to the second (and if we had defined more $3 would be the third etc).

Input validation – we have to be more strict

Often we need to check if the data inputed or read from a file matches a definition so that we know its valid. But for this to work we need to ensure that our expressions only match a valid input. Many expressions of convenience are defined too loose. If we are to use them for input validation we need to harden them.

The pattern we used in an earlier example neatly broke down a valid IP address. But it wasn’t very strict and there are many combinations that would have matched that aren’t valid IP addresses. 999.999.999.999 is not a valid IPv4 address but it would have matched our pattern (@”(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})”). So we couldn’t have used it for testing for a valid IP address.

So what is a valid match? We need to define this first.

A valid IP address range is from 0.0.0.0 to 255.255.255.255 (with each column being represented by a byte).

At this point there are two things we can do: we can validate the results returned by our expressions with a few additional lines of C# code or we modifying our regular expression to become stricter. As this post is about regular expressions we will modify our expression to match only valid IP addresses.

How do we define valid ? 0,9,10,19,100,199,200,249,255 are all valid inputs for each column. 300 isn’t valid, and neither is 299. To keep things simple, we don’t allow 09 as a valid input.

• Single digit: 0 – 9 :   [0-9]
• Double digit: 10 – 99: [1-9][0-9]
• Triple digit 1:  100 – 199:  1[0-9]{2}
• Triple digit 2: and 200 – 249:  2[0-4][0-9]
• Triple digit 3: 250 – 255 25[0-5]

The single ([0-9])and double digit ([1-9][0-9]) combinations can be combined into: [1-9]?[0-9]. (Read as: The first 1-9 is optional, occurs 0 or 1 time)

So a single column can be defined as: (([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.) Note the “.” at the end.

On the final column we do not need a “dot”. We can save some space by repeating the first expression three times, but we need to write out the fourth in full. Thus our expressions becomes: ([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.{3}([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])

Not exactly easy to read, but lets test to see if it works as expected. The following example program tries all column combinations from 0-999

using System;
using System.Text.RegularExpressions;

namespace RegularExpression
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            string IPTestExp = @"(([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|255[0-5])\.){3}([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])";

            for (int Lp = 0; Lp < 999; Lp++)
            {
                string IPAddress = String.Format("{0}.{0}.{0}.{0}",Lp);

                if (Regex.Match(IPAddress,IPTestExp).Success)
                    Console.WriteLine("{0} is valid",IPAddress);
                else
                {
                    Console.WriteLine("{0} is invalid",IPAddress);
                    break;
                }
            }
          }
    }
}

For brevity the program ends at the first invalid combination. If we had let it run it would have shown 256-999 as invalid.

This took a bit of work but we now have a single line test to see if a string is a valid IPv4 address.

Concluding

This ends the second post in this series. In the next post I will look at some advanced regular expression topics.

If you would like to read more on the theory behind regular expressions have a look at the first post in the series: Regular Expressions in C# – The Basics

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This is a post from Martijn's C# Coding Blog.

One of the most common coding tasks is to take an input, munch it around and turn it into something different altogether. Are you looking for FedEx numbers in a text file? Do you want to replace “love” with “hate” in your source files? Is a string a valid e-mail address? Problems like these can be solved by applying regular expressions, or “regex” for short.

Introduction

This post explores the basic theory of expressions. If you are already familiar with them and want to know how to use them in your own C# programs have a look at the next post “Regular Expressions in C# – Practical Applications”

Expressions offer a method of describing and testing for particular combinations of characters in a string. A simple regular expression can often save you from having to write many lines of regular code.

• Are you looking for the characters “car”  in “cartoon”, “carbonate” or  “carton” ?
• Do you want to only match when the word “car” is standing by itself as in  “car sales for 2009″ ?
• Or only return true when the car is red or blue ? “blue car”/ “red car” / “green car”

In C# expressions are provided by the RegEx class in the System.Text.RegularExpressions namespace.

The expressions themselves are more or less standard between computer languages. You can often take an expression from another language and with a little or no work apply them to your C# code. If you are not familiar with them yet you should consider learning to use them.

Regular Expressions to the rescue

What can you use regular expressions for?

• Data capture: split a string into multiple fields which you can manipulate. 13-Jan-2006 becomes (day,month,year)
• Data input validation: Check if the input followed the required formatting rules. For example test if a valid telephone number was entered.
• String comparison: Does A exist in B?
• String replacement: Replace “foo” with “bar”
• Code size reduction: One line of regular expression code can replace large amount of dedicated code

When not to use regular expressions?

Don’t use them when speed is of the essence. Expressions have a serious drawback in that they can be slow to execute. If you are concerned about optimizing a part of your code it can be worthwhile to write your own replacement. In a previous post I noticed that a simple string replacement routine was 40 times faster than the regular expression equivalent.

The basics

To understand expressions we need a little bit of theory. This bit explains all the main operators and how to use them.

Literal characters

The most basic expression contains a single character. If we define “c” as the expression and test it against “car company” it will match against the “c” in “car”. If we ask the RegEx class to search again it will match against the “c” in “company”.

Several characters have a special meaning: ?, +, *, \, [, ( , ), ], {, }, . (dot) and ^

If we want to include them we need to escape them first using a backslash:

• 10 * 10 = 100 wrong
• 10 \* 10 = 100 ok

Normally when parsing strings C# will try to break down escaped sequences such as \n,\r etc. Expression statements usually contain many backslash operators. By adding the “@” string literal the compiler will not inspect the string too much and take it literally instead.

Character Sets

Character sets allow us to limit the characters that can match. Say for example we want to use just the numbers 0-9: [0-9] , or the characters a-z & A-Z: [a-zA-Z]. A character set only matches against a single character, so the following doesn’t work: “c[a-z]kie” matches against “cokie” but not “cookie”.

You can also define your own sets. If you are matching a date, a date separator can be a defined as a space, dash or slash: [ -/]

Many character sets are used so often that they have been given their own shorthands:

• \w matches any word character [a-z,A-Z]
• \s matches any whitespace (space, tab)
• \d matches against any digit [0-9]

For a longer list of the available short hands have a look at my C# Regular Expression Cheat Sheet .

The Dot is special

The dot “.” matches against any character, except for line breaks. You should use it sparingly as it can introduce unwanted results. Often it is better to be more specific, using \w or \d, or a character set that limits the set of possible characters.

• “g..gle” matches “google”, “gaagle”,”g%$gle” and much more.
• “\d\d.\d\d.\d\d” matches a valid date such as “12-08-99″  and “12/08/99″ but also to an invalid date: “12508799″

Creating alternatives using the boolean “or”

A vertical bar separates (|) alternatives, so “red|blue car” would match either a red or blue car. Written in C# code:

if (Regex.Match(“blue car”,”blue|red car”).Success)
Console.WriteLine(“Matches!”);

You can add as many alternatives as you would like, so “red|blue|purple|yellow car” are all possible.

Grouping with parentheses ()

Parentheses () make it easier to group things  together. So if you would like to match for either “color” or “colour” you could write the word “color” (or “colour”) as one of:

• col(o|ou)r
• (color|colour)

Repetition

A repetition quantifier specifies how often a preceding element is allowed to repeat.

To give some examples:

• \d{1,3} reads as “a decimal digit (0-9)”, minimum of 1, maximum of 3
• [az]+ reads as “one or more of a-z”, “abc” matches, and so does “axxxz”

In the following example “aab” matches, but so does “aaab”.

Repetition is useful for testing if an input matches a required pattern. If you need to test for a telephone number formatted as : XXX-XXXX you could write this as \d{3}[-]\d{4}.

Lazy and Greedy matching

All the above repetition operators are “greedy”, they match to the longest possible string they can find.

• a[bz]+z against “abcbzcdze” returns “abcbzcdz“
• <a.+> against "<a href='index.php'>Beginning</a>" matches everything, instead of just the opening <a href"">.

To avoid this we can apply “lazy” matching instead. In a lazy match, as soon as it finds a match the parser stops and returns the result. You can make a match lazy by simply adding a question mark:

• a[bz]+?z against “abcbzcdze” returns “abcbz“
• <a.+?> against "<a href='index.php'>Beginning</a>" returns <a href='index.php'>.

Anchoring

All the above examples didn’t care where in the string the match was made. You could also use them repeatedly to find more instances of the match in the input string. Anchoring allows you to match only those strings that are close to the beginning and/or end.

• ^string reads as: only match if “string” is at the beginning of the input. The “^” indicates the beginning. So “string of wool” matches, but “woolly string” doesn’t.
• string$ reads as: only match if “string” is at the end of the input. Here the “$” indicates the end. In this case “string of wool” can’t match, but “woolly string” can.
• $string^ reads as: only match if “string” is the whole pattern. The “s” comes as the first character, and the “g” as the last. So only “string” can match this pattern.

This ends the theoretical introduction to Regular Expressions — see also the next post “Regular Expressions in C# – Practical Applications” .

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Escape Characters

Alternation

Substitutions

Word boundaries

Quantifiers

This is a post from Martijn's C# Coding Blog.

• The ReplaceFirst method replaces the first occurrence of “needle” in a string and replaces it with “replacement”.
• The ReplaceAll function is similar: it steps through the string modifying it each time it finds “needle” and replaces it. To avoid a possible infinite loop it first checks whether “needle” is equivalent to “replacement”.

using System;
using System.Collections;

namespace StringItems
{
        static class StringExt
        {
                public static string ReplaceFirst(this string haystack, string needle, string replacement)
                {
                        int pos = haystack.IndexOf(needle);
                        if (pos < 0) return haystack;

                        return haystack.Substring(0,pos) + replacement + haystack.Substring(pos+needle.Length);
                }

                public static string ReplaceAll(this string haystack, string needle, string replacement)
                {
                        int pos;
                        // Avoid a possible infinite loop
                        if (needle == replacement) return haystack;
                        while((pos = haystack.IndexOf(needle))>0)
                                haystack = haystack.Substring(0,pos) + replacement + haystack.Substring(pos+needle.Length);
                        return haystack;
                }

        }
}

Both methods are implemented using a class extension. (for more on creating class extensions see also Finding all occurrences of a string within another string) After you include these methods into your project you can call them directly from any string instance:

string myString = “Hello World”;
string myModifiedString = myString.ReplaceFirst(“World”,”People”);
Console.WriteLine(“{0}”,myModifiedString); // Writes: “Hello People”

An example use of the ReplaceAll method:

string myString = “boo foo is not foo boo or foo boo foo”;
string myModifiedString = myString.ReplaceFirst(“boo”,”goo”);
Console.WriteLine(“{0}”,myModifiedString); // Writes: “goo foo is not foo goo or foo goo foo”;

Why not just use a regular expression?

If you are familiar with the RegEx class in C# you can easily write a regular expression to achieve the same string replacement result:

using System.Text.RegularExpressions;
Regex regex = new Regex(“boo”);
string result = regex.Replace(“boo foo is not foo boo or foo boo foo”, “goo”);

Regular expressions are flexible and if you do anything more complex than just a basic string replacement they are your only choice. But they come at a hefty performance price. To run a regular expression it needs to be compiled first and then executed. The .NET runtime caches the expression for performance but using a regular expression for string replacement is still much slower.

How much slower are regular expressions for string replacement?

In an earlier post I described the Stopwatch class in System.Diagnostics. It is ideal for a little benchmark testing — so lets compare my string replacement methods with the build-in regular expression library:

string haystack = "boo foo is not foo boo or foo boo foo";
string result;
Stopwatch sw = Stopwatch.StartNew();
for (int Lp = 0; Lp < 100000; Lp++)
result = regex.Replace($haystack, "goo");
sw.Stop();
Console.WriteLine("Time used (float): {0} ms",sw.Elapsed.TotalMilliseconds);

And the same for the string replacement functions:

string haystack = "boo foo is not foo boo or foo boo foo";
string result;
Stopwatch sw = Stopwatch.StartNew();
for(int Lp = 0; Lp < 100000; Lp++)
result = haystack.ReplaceAll("boo","goo");
sw.Stop();
Console.WriteLine("Time used (float): {0} ms",sw.Elapsed.TotalMilliseconds);

The regular expression code needed 1100ms , whereas the string replacement code needed just 27ms. So for this particular example, the string replacement was 40 times faster than a regular expression.

This is a post from Martijn's C# Coding Blog.