PowerShell has some very good regular expression facilities. I have never used regular expressions enough to retain a level of knowledge that allows me to just break out and write powerful ones unassisted. I keep a bookmark handy to regular-expressions.info whenever I need to produce more than just the basics.
The interactive nature of PowerShell is great in not only testing expressions for use within PowerShell but testing for other situations as well. The -match keyword is the PowerShell comparison operator that handles all the magic.
If you are testing simple string matches you can simply use:
PS> "The quick brown fox jumps over the lazy dog" -match "\b[Tt]he" True
If you are testing a whole file you can use something like this:
sample.txt: Monday: Sunny Tuesday: Cloudy Wednesday: Clear and Cold Thursday: Sunny Friday: Partly Cloudy Saturday: Windy Sunday: Clear
PS> (Get-Content sample.txt) -match "\bSun" Monday: Sunny Thursday: Sunny Sunday: Clear
PS> (Get-Content sample.txt) -match "^Sun" Sunday: Clear
These are simple examples but you can see how easy it is to test regular expressions with PowerShell whether they are for use in your PowerShell work or not.