-Contains operator doesn’t do substring comparisons and the match must be on a complete string and is used to search collections.
From the documentation you linked to:
-Contains Description: Containment operator. Tells whether a collection of reference values includes a single test value.
In the example you provided you’re working with a collection containing just one string item.
If you read the documentation you linked to you’ll see an example that demonstrates this behaviour:
PS C:\> "abc", "def" -Contains "def" True PS C:\> "Windows", "PowerShell" -Contains "Shell" False #Not an exact match
I think what you want is the
"12-18" -Match "-"
Important: As pointed out in the comments and in the linked documentation, it should be noted that the
-Match operator uses regular expressions to perform text matching.