I have a text file "c:zz.txt" of which an extract is as follows:

#N ABSA ALL ROUNDER FoF
#D UT-ABSAAG
#P 20190215 393.83 393.83 0.00

#N ABSA BALANCED FUND
#D UT-ABSABA
#P 20190215 432.28 432.28 0.00

#N COMMUNITY GILT
#D UT-COM-G
#P 20190215 151.21 151.21 8.59

I would like to export this to a .TXT file with following format:

UT_Name,UT_Code,Date,Value1,Value2,Vol
ABSA ALL ROUNDER FoF,UT-ABSAAG,20190215,393.83,393.83,0
ABSA BALANCED FUND,UT-ABSABA,20190215,432.28,432.28,0
COMMUNITY GILT,UT-COM-G,20190215,151.21,151.21,8.59

My code below

clear-Host
get-content -raw "c:zz.txt" | % $_ -replace '(#N)',"`r`n" |% #P)',','|Set-Content ZZ1.txt

Output:

ABSA ALL ROUNDER FoF, UT-ABSAAG, 20190215 393.83 393.83 0.00
ABSA BALANCED FUND, UT-ABSABA, 20190215 432.28 432.28 0.00
COMMUNITY GILT, UT-COM-G, 20190215 151.21 151.21 8.59

Issue:
The problem turns out to be that dataset "#P" in original dataset is fixed length which means I can't simply replace all whitespaces, with commas as this will also affect field names "#N" and "#D" above which I dont want to affect.

How do I selectively replace white spaces?

this uses named capture groups to get the items and then exports it to a CSV file.

# fake reading in a raw text file
# in real life, use Get-Content -Raw
$InStuff = @'
#N ABSA ALL ROUNDER FoF
#D UT-ABSAAG
#P 20190215 393.83 393.83 0.00

#N ABSA BALANCED FUND
#D UT-ABSABA
#P 20190215 432.28 432.28 0.00

#N COMMUNITY GILT
#D UT-COM-G
#P 20190215 151.21 151.21 8.59
'@

# split into blocks, trim unwanted whitespace, filter out the blank block
$SplitInStuff = ($InStuff -split '#N').Trim().Where($_)

$Results = foreach ($SIS_Item in $SplitInStuff)
 
 $Null = $SIS_Item -match '(?sm)(?<UT_Name>^.+$).*#D (?<UT_Code>.+).*#P (?<Date>d+)s+(?<Value1>[0-9.]+)s+(?<Value2>[0-9.]+)s+(?<Vol>[0-9.]+)'
 [PSCustomObject]@
 # the ".Trim()" was needed to remove leftover EOL/NewLine/space chars
 UT_Name = $Matches.UT_Name.Trim()
 UT_Code = $Matches.UT_Code.Trim()
 Date = $Matches.Date.Trim()
 Value1 = $Matches.Value1.Trim()
 Value2 = $Matches.Value2.Trim()
 Vol = $Matches.Vol.Trim()
 
 

$Results |
 Export-Csv -LiteralPath "$env:TEMPmilkywaypizza_ProductInfo.csv" -NoTypeInformation

content of the CSV file ...

"UT_Name","UT_Code","Date","Value1","Value2","Vol"
"ABSA ALL ROUNDER FoF","UT-ABSAAG","20190215","393.83","393.83","0.00"
"ABSA BALANCED FUND","UT-ABSABA","20190215","432.28","432.28","0.00"
"COMMUNITY GILT","UT-COM-G","20190215","151.21","151.21","8.59"

Other method :

$Data = @'
#N ABSA ALL ROUNDER FoF
#D UT-ABSAAG
#P 20190215 393.83 393.83 0.00

#N ABSA BALANCED FUND
#D UT-ABSABA
#P 20190215 432.28 432.28 0.00

#N COMMUNITY GILT
#D UT-COM-G
#P 2019021 151.21 151.21 8.59
'@

#template for learn schema
$template=@'
#N Vol_ABSA_ALL_ROUNDER_FoF*:ABSA ALL ROUNDER FoF
#D UT-UT_Name:ABSAAG
#P Date:20190215 Value1:393.83 Value2:393.83 Vol:0.00

#N Vol_ABSA_ALL_ROUNDER_FoF*:ABSA ALL ROUNDER FoF 2
#D UT-UT_Name:ABSAAG2
#P Date:20190216 Value1:393.83 Value2:393.83 Vol:0.00
'@

$Data | ConvertFrom-String -TemplateContent $template | export-csv "c:tempresult.csv" -NoType

How do I selectively replace runs of multiple whitespace characters?

E.g., by only selecting multiple spaces with a quantifier -replace " 2,",','

The following script based on your one liner:

## Q:Test2019328SO_55387785.ps1
$FileIn = '.zz.txt'
$FileOut= '.zz1.txt'

 Set-Content $FileOut -Value "UT_Name,UT_Code,Date,Value1,Value2,Vol"
(Get-Content $FileIn -raw) -replace "(`r?`n)?#N " -replace "`r?`n(#D|#P) | 2,",',' |
 Add-Content $FileOut

yields this output:

> Get-Content .ZZ1.txt
UT_Name,UT_Code,Date,Value1,Value2,Vol
ABSA ALL ROUNDER FoF,UT-ABSAAG,20190215,393.83,393.83,0.00
ABSA BALANCED FUND,UT-ABSABA,20190215,432.28,432.28,0.00
COMMUNITY GILT,UT-COM-G,20190215,151.21,151.21,8.59