using a partial variable as part of regular expression string - regex

Question

I'm trying to loop through an array of alarm codes and use a regular expression to look it up in cpp code. I know my regex works when I hard code a value in and use double quotes for my regex, but I need to pass in a variable because it's a list of about 100 to look up with separate definitions. Below is what I want to use in general. How do I fix it so it works with $lookupItem instead of hard-coding "OTHER-ERROR" for example in the Get-EpxAlarm function? I tried single quotes and double quotes around $lookupItem in the $fullregex definition, and it returns nothing.
Function Get-EpxAlarm{
[cmdletbinding()]
Param ( [string]$fileContentsToParse, [string]$lookupItem)
Process
{
$lookupItem = "OTHER_ERROR"
Write-Host "In epx Alarm" -ForegroundColor Cyan
# construct regex
$fullregex = [regex]'$lookupItem', # Start of error message########variable needed
":[\s\Sa-zA-Z]*?=", # match anything, non-greedy
"(?<epxAlarm>[\sa-zA-Z_0-9]*)", # Capture epxAlarm Num
'' -join ''
# run the regex
$Values = $fileContentsToParse | Select-String -Pattern $fullregex -AllMatches
# Convert Name-Value pairs to object properties
$result = $Values.Matches
Write-Host $result
#Write-Host "result:" $result -ForegroundColor Green
return $result
}#process
}#function
#main code
...
Get-EpxAlarm -fileContentsToParse $epxContents -lookupItem $item
...
where $fileContentsToParse is
case OTHER_ERROR:
bstrEpxErrorNum = FATAL_ERROR;
break;
case RI_FAILED:
case FILE_FAILED:
case COMMUNICATION_FAILURE:
bstrEpxErrorNum = RENDERING_ERROR;
break;
So if I look for OTHER_ERROR, it should return FATAL_ERROR.
I tested my regular expression in regex editor and it works with the hard-coded value. How can I define my regex so that I use the parameter and it returns the same thing as hard-coding the parameter value?

Answer 1

I wouldn't recommend trying to construct a single regular expression to do complex source code parsing - it gets quite unreadable really quickly.
Instead, write a small error mapping parser that just reads the source code line by line and constructs the error mapping table as it goes along:
function Get-EpxErrorMapping {
param([string]$EPXFileContents)
# create hashtable to hold the final mappings
$errorMap = #{}
# create array to collect keys that are grouped together
$keys = #()
switch -Regex ($EPXFileContents -split '\r?\n') {
'case (\w+):' {
# add relevant key to key collection
$keys += $Matches[1] }
'bstrEpxErrorNum = (\w+);' {
# we've reached the relevant error, set it for all relevant keys
foreach($key in $keys){
$errorMap[$key] = $Matches[1]
}
}
'break' {
# reset/clear key collection
$keys = #()
}
}
return $errorMap
}
Now all you need to do is call this function and use the resulting table to resolve the $lookupItem value:
Function Get-EpxAlarm{
[CmdletBinding()]
param(
[string]$fileContentsToParse,
[string]$lookupItem
)
$errorMap = Get-EpxErrorMapping $fileContentsToParse
return $errorMap[$lookupItem]
}
Now we can get the corresponding error code:
$epxContents = #'
case OTHER_ERROR:
bstrEpxErrorNum = FATAL_ERROR;
break;
case RI_FAILED:
case FILE_FAILED:
case COMMUNICATION_FAILURE:
bstrEpxErrorNum = RENDERING_ERROR;
break;
'#
# this will now return the string "FATAL_ERROR"
Get-EpxAlarm -fileContentsToParse $epxContents -lookupItem OTHER_ERROR