The -match operator is frequently used in scripts however not everyone seems to understand how it really works. It can be a really dangerous filter operator.
Let’s first create some sample data:
$list = 'ServerName, Location, Status Test1, Hannover, Up Test2, New York, Up Test11, Sydney, Up' | ConvertFrom-Csv $list
The result is a list of hypothetical servers:
ServerName Location Status ---------- -------- ------ Test1 Hannover Up Test2 New York Up Test11 Sydney Up
Let’s assume you want a PowerShell script to pick a server from the list and manipulate it, i.e. power it down:
# server to work with: $filter = 'Test2' # pick filter from list: $list | Where-Object ServerName -match $filter
All seems to work just fine:
ServerName Location Status ---------- -------- ------ Test2 New York Up
However, -match is expecting a regular expression and not just plain text. Plus it returns $true if the match was found anywhere in the text. When you change $filter to “Test1” to pick server “Test1”, here is the result:
ServerName Location Status ---------- -------- ------ Test1 Hannover Up Test11 Sydney Up
You would have accidentally picked two servers because “Test11” also contains the text “Test1”.
Even worse: if for some dumb reason $filter is left blank, you would select everything – because “blank” is matching anything. Try yourself and set $filter to value ‘’.
Be very careful when picking comparison operators, and be extra careful with -match. In the example above, the -eq operator (equals) would have been much more appropriate, and if you must use wildcards, -like would be a lot more explicit because it requires an explicit “*” wildcard if you really want to compare only against parts of the value.
