First, you must understand what a simple expression is. A simple expression represents a specific value that you want to match with a condition or exception. An example of a simple expression is the title of a document that your organization does not want to be distributed outside the organization. A piece of data in an e-mail message must exactly match a simple expression to satisfy a condition or exception in transport rules.

A regular expression is a concise and flexible notation for finding patterns of text in a message. The notation consists of two basic character types: literal (normal) text characters, which indicate text that must exist in the target string, and metacharacters, which indicate or control how the text can vary in the target string. You can use regular expressions to quickly parse e-mail messages to find specific character patterns.

The ability to find patterns of text in an e-mail message enables you to match predicates against data in messages that can change dynamically. Examples of such data are Social Security Numbers (SSN) and patent numbers. You cannot reasonably match this data with a simple expression because a simple expression requires that you enter every variation of the value that you want to detect. By using regular expressions, you can configure the predicate to search for the pattern of SSNs or patent numbers in a message.

You can use regular expressions in any condition or exception rule predicate, except "is a delivery report". For more information about which predicates accept regular expression pattern matching, see Transport Rule Predicates.

In the Exchange Management Shell, you can use regular expressions in any predicate that accepts the Patterns predicate property. In the Exchange Management Console, you can use regular expressions with any condition or exception that contains the words with text patterns. Table 1 lists all the pattern strings that you can use to create a pattern-matching regular expression.

Caution:
You must carefully test the regular expressions that you construct to make sure that they yield the expected results. An incorrectly configured regular expression could yield unexpected matches and cause unwanted transport rule behavior. Test your regular expressions in a test environment before you implement them in production.

Table 1   Pattern strings

By using Table 1, you can construct a regular expression that matches the pattern of the data that you want to match. Working from left to right, examine each character or group of characters in the data that you want to match. Read the description of each pattern string to determine how it is applied to the data that you are matching. Then, determine which pattern string in Table 1 represents that character or group of characters, and add that pattern string to the regular expression. When you are finished, you will have a fully constructed regular expression.

For example, the following regular expression matches North American telephone numbers in the formats 425 555-0100 and 425.555.0100:

425(\s|.)\d\d\d(-|.)\d\d\d\d

You can expand on this example by adding the telephone format (425) 555-0100, which uses parentheses around the area code. The following regular expression matches all three telephone number formats:

(\\()*\d\d\d(\\)|\s|.)\d\d\d(-|.)\d\d\d\d

You can analyze the previous example as follows:

• (\\()*   This portion makes the first parentheses optional. Because the closing parenthesis is also a regular expression delimiter, it must be escaped by using two backslashes \\. The surrounding (()) parentheses group the \\( characters together so that the wildcard character * can act upon the \\( characters to make them optional.
  
• \d\d\d   This portion requires that exactly three numeric digits appear next.
  
• (\\)|\s|.)   This portion requires that an opening parenthesis, a space, or a period exist after the three-digit number. Each character-matching string is contained in the grouping delimiters and is separated by the pipe character. This means that only one of the specified characters inside the grouping delimiters can exist in this location in the string that is being matched.
  
• \d\d\d   This portion requires that exactly three numeric digits appear next.
  
• (-|.)   This portion requires that either a hyphen or period exists after the three-digit number. Because the hyphen and period exist in the grouping delimiters, only one of the two characters can exist in this location in the string that is being matched.
  
• \d\d\d\d   This portion requires that exactly four numeric digits appear next.
  

The following example shows how you can use regular expressions when you create a new rule in the Exchange Management Shell:

1. Run the following commands:

   $Condition = Get-TransportRulePredicate SubjectMatches
   $Condition.Patterns = @("\d\d\d-\d\d-\d\d\d\d")
   $Action = Get-TransportRuleAction RejectMessage
   $Action.RejectReason = "The transmission of Social Security Numbers is prohibited."
   New-TransportRule -Name "Social Security Number Block Rule" -Condition $Condition -Action $Action 

2. Run the following command to view the new transport rule:

   Get-TransportRule "Social Security Number Block Rule" | Format-List

When this Get-TransportRule command is run, the following information is displayed:

Identity           : Social Security Number Block Rule,753ed939-1227-4b2a-a8e0-ec49b0615f30
Name               : Social Security Number Block Rule
RuleCollectionName : Transport
Priority           : 0
Comments           :
ManuallyModified   : False
Conditions         : {SubjectMatches}
Exceptions         :
Actions            : {RejectMessage}
State              : Enabled
IsValid            : True
ObjectState        : Unchanged

For detailed syntax and parameter information about each command, see the following topics:

• New-TransportRule
  
• Get-TransportRule
  
• Get-TransportRulePredicate
  
• Get-TransportRuleAction
  

For more information about transport rules, see the following topics:

• How to View a Transport Rule
  
• How to Create a New Transport Rule
  
• How to Modify a Transport Rule
  
  • How to Modify Transport Rule Conditions and Exceptions in the Exchange Management Shell
    
  • How to Modify Transport Rule Actions in the Exchange Management Shell
    
• How to Remove a Transport Rule
  
• How to Configure a Disclaimer
  
• Transport Rule Predicates
  
• Transport Rule Actions