Match a Sub-string with a Regular Expression in TestStand

This article demonstrates how to use a regular expression in TestStand to identify a pattern within a larger string. This approach is useful when the regular expression matches only part of the string rather than the entire content. TestStand's FindPattern() function can be used to either:

1. Verify whether a match exists and return a Pass/Fail result.
2. Extract and return the matching substring.

These techniques are useful when parsing serial numbers, instrument responses, log files, or any text containing embedded data.

Example Scenario

Assume a device returns the following string:

Device SN: ABC12345

And you want to identify the serial number using the following regular expression:

ABC\d+

Expected match:

ABC12345

Since the pattern only matches a portion of the string, the FindPattern() function can be used to locate the matching substring.

Method 1: Return a Pass/Fail Result

Use this approach when you only need to determine whether the specified pattern exists within the string.

Steps

1. Create the following Local Variables:
   1. SerialNumber (String): The string to search.
   2. Pattern (String): The regular expression to match.
   3. PatternLength (Number): Stores the length of the matched pattern. This is an output variable required by the FindPattern() function.
2. Add a Pass/Fail Test step to the sequence.
   1. Use the <None> adapter for this test.
3. Open the Data Source tab and enter:
   
   FindPattern(Locals.SerialNumber, Locals.Pattern, 0, True, Locals.PatternLength) >= 0
   
   
4. Run the sequence.
5. Review the step result:
   1. Pass indicates the pattern was found.
   2. Fail indicates the pattern was not found.

Testing the sequence - Pass case

1. Manually set the input variables as shown below:
   
   

   Variable	Value
   Locals.SerialNumber	Device SN: ABC12345
   Locals.Pattern	ABC\d+

   
   
   
2. Place a breakpoint right after your Pass/Fail step
3. Click Execute>>Run MainSequence.
4. See that your step passed.
   
   
    

Testing the sequence - Fail Case

1. Manually set the input variables as shown below:
   
   

   Variable	Value
   Locals.SerialNumber	Device SN: AXC12345
   Locals.Pattern	ABC\d+

   
   
   
2. Place a breakpoint right after your Pass/Fail step
3. Click Execute>>Run MainSequence.
4. See that your step failed.
   
    

Method 2: Return the Matched String

Use this approach when you need to retrieve, display, or further process the matching text.

Steps

1. Create the following Local Variables:
   1. SerialNumber (String): The string to search.
   2. Pattern (String): The regular expression.
   3. FoundIndex (Number): Stores the starting location of the match.
   4. PatternLength (Number): Stores the length of the matched string.
2. Add a Statement step.
3. Enter the following statement:
   
   Locals.FoundIndex = FindPattern(Locals.SerialNumber, Locals.Pattern, 0, True, Locals.PatternLenght)
   
   
4. Open a Message Popup step.
5. Open the Text and Buttons tab.
6. Enter the following Message Expression:
   
   "The macthed string is " + Mid(Locals.SerialNumber, Locals.FoundIndex, Locals.PatternLenght)

Testing the sequence

1. Manually set the input variables as shown below:
   
   

   Variable	Value
   Locals.SerialNumber	Device SN: ABC12345
   Locals.Pattern	ABC\d+

   
   
   
2. Remove any breakpoints that may exist in your sequence.
3. Click Execute>>Run MainSequence.
4. The Message Popup displays the matching substring returned from the original string.
   
    

How It Works

The FindPattern() function searches a string for a regular expression match and returns the starting index of the first match. The function can also populate a variable with the length of the matched text. This information can then be used with the Mid() function to extract the matching substring from the original string.