PowerShell script to concatenate regex and variable - regex

Question

I want to write PowerShell script and regex to audit several network devices configuration files for compliance. Some devices are configured one management vlan while others have multiple different management vlans. Examples below
Config1:
VLAN Name Status Ports
1 default active
100 12_NET_MGMT_VLAN active Gi1/2
Config2:
VLAN Name Status Ports
1 default active
88 100_MGMT-VLLAN active Gi8/1
100 12_Net_MGMT_VLAN active
If I hard code the regex pattern like this $regex_pattern = "^\d{1,3}\s+.*MGMT.*", I got the corrected output as expected
Config1 12_NET_MGMT_VLAN
Config2 100_MGMT_VLAN
Config2 12_Net_MGMT_VLAN
Instead of hard-code the regex pattern, I want to use the Read-Host cmdlet and ask a user to enter the word "MGMT" and store it in a variable $Mgmt, then concatenate with a regex pattern to create a dynamic regex pattern, like this:
$Mgmt = Read-Host "Enter a word pattern to find a management vlan: "
For example, a user type in MGMT, and then I created a dynamic regex pattern as below:
$regex_pattern = "^\d{1,3}\s+.*"+$Mgmt+"_.*"
$regex_pattern = "^\d{1,3}\s+.*"+[regex]::escape($Mgmt)+".*"
None of the results came out correct
If anyone has a solution, please help. Thx

Answer 1

If we are to assume that VLAN names cannot contain spaces, you can use \S (non-space) as an anchor character. Using the subexpression operator $(), you can evaluate an expression within a string.
# Simulating a vlan config output
$Config = #'
VLAN Name Status Ports
1 default active
88 100_MGMT-VLLAN active Gi8/1
100 12_Net_MGMT_VLAN active
'# -split '\r?\n'
# Using value MGMT here when prompted
$Mgmt = Read-Host "Enter a word pattern to find a management vlan"
$regex = "\S*$([regex]::Escape($Mgmt))\S*"
[regex]::Matches($Config,$regex).Value
Output:
100_MGMT-VLLAN
12_Net_MGMT_VLAN
Note that simple variable references like $Mgmt will expand properly within surrounding double quotes, e.g. "My VLAN is $Mgmt".
You could take this in a different direction and create a custom object from your output. This would enable you to use filtering via Where-Object and member access (.Property) to retrieve target data. This again assumes values don't contain spaces.
# Simulating a vlan config output
$Config = #'
VLAN Name Status Ports
1 default active
88 100_MGMT-VLLAN active Gi8/1
100 12_Net_MGMT_VLAN active
'# -split '\r?\n'
$Mgmt = Read-Host "Enter VLAN Name"
# Replacing consecutive spaces with , first
$ConfigObjs = $Config -replace '\s+',',' | ConvertFrom-Csv
$ConfigObjs
Output:
VLAN Name Status Ports
---- ---- ------ -----
1 default active
88 100_MGMT-VLLAN active Gi8/1
100 12_Net_MGMT_VLAN active
Now you have properties that can be referenced and access to other comparison operators so you don't always need to use regex.
($ConfigObjs | Where Name -like "*$Mgmt*").Name
Output:
100_MGMT-VLLAN
12_Net_MGMT_VLAN