Need to get the string in between two patterns in PowerShell

I would start to create an array defining your keywords:

$keywords = @(
    '<dt>Commodity Name</dt>'
    '<dt>IP address</dt>'
    '<dt>Commodity Serial #</dt>'
    '<dt>Client IP</dt>'
    '<dt>Client Logged In As</dt>'
    '<dt>User is member of</dt>'
)

Now you can join the keywords by an | to use it with the Select-String cmdlet:

$file = 'C:\Users\rpm\Desktop\sample.txt'
$content = Get-Content $file
$content | Select-String -Pattern ($keywords -join '|')

This will give you the line number of each matched keyword. Now you can iterate over the result, access the next line by index and crop the <dd> pre and </dd> postifx:

ForEach-Object {
        [regex]::Match($content[$_.LineNumber], '<dd>(.+)</dd>').Groups[1].Value
    }

Regex:

Output:

Grocery
XXX.XXX.XXX.XXX port 8000
0055500000
xxx.xxx.xxx.xxx
rpm123
BP-RPM\COMD_CSO_ITM-AVAI_Def,BP-RPM\user

Finally you have to join the result by | to get the desired output. Here is the whole script:

$keywords = @(
    '<dt>Commodity Name</dt>'
    '<dt>IP address</dt>'
    '<dt>Commodity Serial #</dt>'
    '<dt>Client IP</dt>'
    '<dt>Client Logged In As</dt>'
    '<dt>User is member of</dt>'
)

$file = 'C:\Users\rpm\Desktop\sample.txt'
$content = Get-Content $file

($content | Select-String -Pattern ($keywords -join '|') | 
    ForEach-Object {
        [regex]::Match($content[$_.LineNumber], '<dd>(.+)</dd>').Groups[1].Value
    }) -join ' | '

Output:

Grocery | XXX.XXX.XXX.XXX port 8000 | 0055500000 | xxx.xxx.xxx.xxx | rpm123 | BP-RPM\COMD_CSO_ITM-AVAI_Def,BP-RPM\user