There's too much speculation and little details with no further replies from the OP.

What about avoiding the pattern match and check for valid date values?

DECLARE @searchString varchar(1500);

SET @searchString='03/09/2017 07:52 AM | Lastname, Firstname | Completed - irrelevant...

Why would you try to escape the pipe? It's not a wildcard in t-sql.

Sure, you can read the following article to know the best way to do it.
Cross Tabs and Pivots, Part 1 – Converting Rows to Columns - SQLServerCentral

I suggest that you read this article from Itzik Ben-Gan.
http://www.itprotoday.com/microsoft-sql-server/logical-query-processing-what-it-and-what-it-means-you

You also need to learn about execution plans and the different operators. You'll understand that tables...

What happens if you run the following query?

SELECT YAB
FROM UD_Indiv_Data
WHERE YAB IS NOT NULL
AND TRY_CAST( YAB AS DATETIME) IS NULL;

If...

Is the data coming from different tables?
It seems that the easiest way would be to use both queries as CTEs and use a FULL JOIN between them.
Something like...

I've experienced problems with Intellisense when the connection is slow or the database is too large. Try refreshing the cache and give it some time to do it.
Or you...

Maybe something like this?

UPDATE t SET
  MyAcctNumber = x.MyAcctNumber
FROM #Test t
JOIN (
  SELECT Id, LAG( Id,1,0) OVER( ORDER BY Id) LAGId,...