What is name of a variable within a variable - regex

Question

I'm assigning a series of regex's to vars. Some of the regex values will be the same but unique and be identifiable by the var name itself ($a and $c as example).
#various regex
$a = "([\d]{1,2})"
$b = "([\d]{3})"
$c = $b #Note this has the same regex as $b
$d = "\s[-]\s"
$e = "[_]"
#select the pattern
$patternNum = 4
I then want to be able to concat the vars in different orders to create a larger regex.
Switch ($patternNum){
#create a pattern
1 { $pattern = ($a, $e, $b) }
2 { $pattern = ($a, $d, $b) }
3 { $pattern = ($a, $d, $a, $e, $b) }
4 { $pattern = ($a, $e, $b, $e, $c) }
}
This creates the expanded regex string i'm hoping for
#so i can use full regex pattern later
$selectedPattern = -join $pattern
But I want to be able to associate the var in $pattern with the original var name and not the literal string that's associated with the var (as some strings will be the same)
#find the index of each var and assign to another var so var can be used later to identify position within match
var1 = [array]::IndexOf($pattern, $a) # [0]
var2 = array]::IndexOf($pattern, $b) # [2]
var3 = [array]::IndexOf($pattern, $c) # [2] but i want it to be [4]
The regex which will be used in matching, each match will be used in different strings and in different positions
I thought i'd be able to use scriptblock {} and then convert back to string but that doesn't seem to work. Can anybody think of a way to get each vars original var name or think of a better way of doing this?

Answer 1

Using named captures
Use (? ) syntax to create named captures. Make the name the same as your variable names, e.g.:
$A = '(?<A>\d{3})'
$B = '(?<B>\D{3})'
$string = 'ABC123'
$regex = $B + $A
$string -match $regex
$Matches
Name Value
---- -----
A 123
B ABC
0 ABC123
Now you can correlate the variables to the position they matched in the string like this:
$string.IndexOf($Matches.A)
3
$string.IndexOf($Matches.B)
0

Answer 2

following your code I'll do it like this, but knowing what's is your real need someone can suggest other solution:
$c = $b
$d = "\s[-]\s"
$e = "[_]"
#select the pattern
$patternNum = 4
Switch ($patternNum){
#create a pattern
1 { $pattern = ('$a', '$e', '$b') }
2 { $pattern = ('$a', '$d', '$b') }
3 { $pattern = ('$a', '$d', '$a', '$e', '$b') }
4 { $pattern = ('$a', '$e', '$b', '$e', '$c') }
}
$selectedPattern = -join $pattern
$var1 = [array]::IndexOf($pattern, '$a') # [0]
$var2 = [array]::IndexOf($pattern, '$b') # [2]
$var3 = [array]::IndexOf($pattern, '$c') # [4]
#converting literal to your pattern
$regexpattern = $ExecutionContext.InvokeCommand.ExpandString( -JOIN $pattern )
$regexpattern
([\d]{1,2})[_]([\d]{3})[_]([\d]{3})