Regular Expressions in .NET

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Regular Expressions in .NET

• Ashraya R. Mathur
• CS 795 - .NET Security

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Outline

• Introduction to Regular Expressions
• Regular Expression Syntax
• Validation in ASP.NET
• Regular Expressions in .NET Programming
• Demonstrations
• Conclusion

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What are Regular Expressions?

• Definition
• A Regular Expression is a series of characters
  that are transformed into an algorithm that
  matches and manipulates text
• Allow you to
• Extract, edit, replace, or delete text substrings
  
• Add the extracted strings to a collection in
  order to generate a report
• Are a universally valuable skill applicable in
  .NET, Java, Perl, PHP, JavaScript, and many other
  programming languages

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Common Regular Expression Uses

• Form and Data Validation
• Query-String Validation
• Data Clean-up / Reformatting
• Data search and retrieval
• HTML / XML Information Retrieval
• Parsing Log Files

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Regular Expressions Syntax

• Simple Expressions
• Simplest Regular Expression - the literal string
• Quantifiers
• , which describes "0 or more occurrences,
• , which describes "1 or more occurrences", and
• ?, which describes "0 or 1 occurrence".
• Explicit Quantifiers - x,y, which allow an
  exact number or range to be specified
• Quantifiers always refer to the pattern
  immediately preceding (to the left of) the
  quantifier

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Regular Expressions Syntax(contd)

• Metacharacters
• include the following . ( ) and \
• . matches a single character
• and mark the start and end positions of a
  line of text. Ex aa-zb
• () are used to group an expression. Ex (abc)
• A class of characters from which the pattern
  can match one. Ex a-z, A-Z, 0-9
• indicates an either-or situation Ex abcd
• \ used as an escape character. Ex c\\

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Sample Regular Expressions
Pattern Description
\d5 5 numeric digits, US ZIP code.
(\d5(-\d4)? Same as previous, but more efficient. Optional US ZIP4 format
\w_at_a-z?\.a-z2,3 Simple email validation expression
\d3-\d2-\d4 Social Security Number Validation
\d1,2\/\d1,2\/\d4 Date Format Validation
(\w-\.)\w-(/\w- ./?)? URL Validation
/\.\/ Matches the contents of a C-style comment / /
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Validation in ASP.NET

• RegularExpressionValidator Validation Control
• Allows you to validate inputs by providing a
  regular expression which must match the input.
• The regular expression pattern is specified by
  setting the ValidationExpression property of the
  control.
• Key properties
• ControlToValidate
• ErrorMessage (for the ValidationSummary)
• ltaspRegularExpressionValidator runat"server"
  iddate1 ControlToValidateTextBox1"
  ErrorMessage"Invalid Date" ValidationExpression
  "\d1,2\/\d1,2\/\d4" /gt

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Regular Expressions in .NET Programming

• .NET Base Classes
• Namespace System.Text.RegularExpressions
• Can use from any .NET language
• Implements the Traditional NFA RegEX Engine
• As does Java, Perl, PHP etc..
• Almost all patterns will work the same
• .NET is only one to implement Named Captures

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The RegEx NamespaceSystem.Text.RegularExpressions

• RegEx
• Match
• MatchCollection
• Group
• GroupCollection
• Capture
• CaptureCollection
• RegExCompilationInfo

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The Regex Base Class

• The Regex class represents a single regular
  expression
• It is immutable, which means once you create it,
  you cannot change it
• To create a Regex object in C, you can first
  define it and then instantiate it with the
  regular expression pattern, as shown here

Regex myRegex myRegex new Regex(RegularExpressionPattern)
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The Regex Base Class (Contd)

• Match Searches a given string and returns a
  single Match object for the first text that is
  matched by the regular expression pattern
• Matches Searches a given string and returns a
  MatchCollection object for all locations that are
  matched by the pattern stored in the Regex object
• IsMatch Returns True if the provided string
  contains the pattern
• Split Splits the given string into an array of
  substrings using the regular expression pattern
  as the delimiter
• Replace Replaces any instances of text that
  match the pattern in the Regex object with the
  provided expression

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Demonstration 1

• private void btnRun_Click(object sender,
  System.EventArgs e)
• //Use the RegEx object to determine if there is
  a match here we use a //single RegEx object
  passing in the pattern and option to ignore case
• Regex rxMatch new Regex(txtRegEx.Text,
  RegexOptions.IgnoreCase)
• //determine if there is a match using the user
  input
• bool blnResultrxMatch.IsMatch(txtText.Text)
• //display those results to the user
• MessageBox.Show("The Result is "
  blnResult.ToString(),"RegEx Demo")

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Match and Match Collection

• Allows us to obtain the details of each match
  made via a regular expression
• Match-represents a single match made
• MatchCollection-a collection of Match Objects
• When the Match method of the Regex object is
  used, it returns a Match object that contains the
  matching text
• The MatchCollection object contains a series of
  Match objects, each representing a single
  substring from the string searched

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Demonstration 2

• private void btnRun_Click(object sender,
  System.EventArgs e)
• //Use the RegEx object to determine if there is a
  match here we use a
• //single RegEx object passing in the pattern and
  option to ignore case
• Regex rxMatch new Regex(txtRegEx.Text,
  RegexOptions.IgnoreCase)
• Match mtMatch
• MatchCollection mtCol
• mtMatch rxMatch.Match(txtText.Text)
• mtColrxMatch.Matches(txtText.Text)
• MessageBox.Show("There are " mtCol.Count "
  matche(s) found.","RegEx Demos")

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Demonstration 2 (contd)

• //if there are more than 0 matches, show them
• if (mtCol.Countgt0)
• //use the Match object here
• do
• //we want the match.value and position in the
  string
• MessageBox.Show("Result at position string "
  mtMatch.Index.ToString() " "
  mtMatch.Value.ToString(),"RegEx Demos")
• mtMatchmtMatch.NextMatch()
• while (mtMatch.Success)

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Group and GroupCollection

• Capturing ()
• The captured subsequence may be used later in the
  expression, via a back reference, and may also be
  retrieved from the matcher once the match
  operation is complete
• Non-Capturing (?)
• Named Capture (.NET only) (?ltnamegt)
• Uses names for the captured groups instead of
  numbers
• Substitutions
• Specialized Replace via groups

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Backreferences Advanced Grouping

• Backreferences
• Allows you to match the same characters as a
  previous group
• Match repeated words
• (\ba-zA-Z \b)\s\1
• Advanced Grouping
• Positive Look-Ahead Assertion (?)
• Negative Look-Ahead Assertion (?!)
• Positive Look-Behind Assertion (?lt)
• Negative Look-Behind Assertion (?lt!)
• Non-Backtracking (?gt)

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Replacing Substrings

• The Replace method of Regex is used to replace
  matched portions of a given string with the
  specified replacement.
• Example using backrefrence named capture
• NewDateYMD Regex.Replace( OldDateMDY,
  \b(?ltmonthgt\d1,2)/(?ltdaygt\d1,2)/(?ltyeargt\d2,
  4)\b, year-month-day)

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Demonstration 3

• private void btnCapture_Click(object sender,
  System.EventArgs e)
• //a basic pattern that will capture any word w/
  4 characters
• string strRegExPattern"(A-Za-z4)"
• Regex rxGroups new Regex(strRegExPattern,RegexO
  ptions.IgnoreCase)
• //Match Object-gtUsing a group here
• Match mtGroup rxGroups.Match(txtCapture.Text)
  
• //get all of the groups that exist
• do
• MessageBox.Show(mtGroup.Groups1.Value, "RegEx
  Demos")
• mtGroupmtGroup.NextMatch()
• while (mtGroup.Success)

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Demonstration 3 (contd)

• private void btnNamedCapture_Click(object sender,
  System.EventArgs e)
• //a basic pattern that will capture any word w/ 4
  characters
• //and the ability to use named capturing
• string strRegExPattern"(?ltwordgtA-Za-z4)"
• Regex rxGroups new Regex(strRegExPattern,RegexOp
  tions.IgnoreCase)
• Match mtGroup rxGroups.Match(txtCapture.Text)
• do
• //show the match using the named reference "word"
• MessageBox.Show(mtGroup.Result("word"), "RegEx
  Demos")
• mtGroupmtGroup.NextMatch()
• while (mtGroup.Success)

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Demonstration 3 (contd)

• private void btnBack_Click(object sender,
  System.EventArgs e)
• //Use the RegEx object to determine if there is
  a
• //duplicate word here using the
  (\ba-zA-Z\b)\s\1 pattern
• Regex rxMatch new Regex(txtRegEx.Text,RegexOp
  tions.IgnoreCase)
• //string to replace the text into
• //replace the repeated word /w nothing 1
• string strReplacerxMatch.Replace(txtBack.Text,"
  1")
• //show the results
• MessageBox.Show(strReplace,"RegEx Demos")

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References

• Regular Expression Library http//regexlib.com/
• Regular Expressions Information Website
  http//www.regular-expressions.info/dotnet.html
• Regular Expressions in .NET MSDN Library
• Professional Visual Studio 2005
• Andrew Parsons and Nick Randolph

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Questions?