Is it possible to parse a PowerShell param block by using a regular expression

Question

I search for a regular expression to extract a Powershell param block from a complete PowerShell script.

[CmdletBinding()]
Param(
     [Parameter(Mandatory=$True,Position=1)]
     [string]$computerName,

     [Parameter(Mandatory=$True)]
     [string]$filePath
    )

...

A colleague means that it is not possible to parse this structure by using a regualr expression. He said regular expressions can't count the opening and closing signs (...) inside the param block and that makes it impossible to parse the param block.

I'm sure its possible but it would be a complex regex with a lot of escaped characters. What have you tried and what output are you ultimately trying to achieve? — Mark Wragg, Apr 19 '17 at 08:17
See [.NET regex reference](https://msdn.microsoft.com/en-us/library/az24scfc(v=vs.110).aspx#Anchor_3): you can use balancing group constructs. However the correct approach is to use AST parser ([example](http://stackoverflow.com/a/42548383/3959875)) — wOxxOm, Apr 19 '17 at 08:17
Is there a particular reason _why_ you're even contemplating using regex here instead of just using PowerShell's parser? — Joey, Apr 19 '17 at 08:51
I was recently contemplating the same thing (from the looks of your sample, pretty much the same thing really). But in the end I just decided to use another meta comment for that: `# param: computer, path` for each script. Much easier to extract, more flexibility elsewhere… — mario, Apr 20 '17 at 22:36

score 1 · Answer 1 · edited May 23 '17 at 12:34

Good news: your colleague is wrong.
It is true that a simple regex has issues with nested data, but that is only true if you have unlimited level nesting.
This is explained very nicely in this question: How to match string within parentheses (nested) in Java?

In your case, even a simple regular expression can work:

(?:\[[^\]]+\]\s*)*\$\w+

Working example

This finds $parameters with blocks of [attributes] before them. There is very little nesting involved. This is a naive regex, and it fails if you have comments, strings with brackets, etc. But the regex can be expanded to support these cases.

Some more points:

Practically, of course a parser is better.
Advanced regular expression engines have features like recursion or balancing groups, which make even unlimited-level matching possible.
You want to make sure you are not matching elsewhere in the text, for example inside a string or on commented text. I've mentioned similar edge cases here: Why it's not possible to use regex to parse HTML/XML: a formal explanation in layman's terms

Your example doesn't cover default value of optional parameters — Clijsters, Apr 19 '17 at 08:56
@Clijsters - Right! of course it doesn't, it is not meant to be a complete example. I did say "This is a naive regex, and it fails if you have comments, strings with brackets, etc. But the regex can be expanded to support these cases". This is a *theoretical* is-it-possible question, not a practical one. — Kobi, Apr 19 '17 at 08:59

Is it possible to parse a PowerShell param block by using a regular expression

1 Answers1