How to substitute accented characters when renaming text files using first line in Powershell

I'm trying to batch rename plain text files using the first line of each file. I want to keep only alphanumeric characters in with your help I'm almost there. The only issue is that I need accented characters like é or á to be preserved in a form of their respective not accented characters: e and a (text is in Spanish) or be preserved in the name as they are, not removed. This is what I'm using right now:

Get-ChildItem *.txt | Rename-Item -NewName {
    $firstLine = ($_ | Get-Content -TotalCount 1) -replace '[^a-z0-9 ]'
    '{0}.txt' -f $firstLine
}

Thank you. If possible, please let me know if there is a way to keep the symbol "?" too.

Approach is similar to the one used in this answer, you can use the String.Normalize Method before your regex replacement.

As for not removing ?, you can simply add it to the character range: [^a-z0-9 ?], however it is an invalid character for file names in Windows, thus not used in the renaming code snippet for this answer. You can use [IO.Path]::GetInvalidFileNameChars() to get the list of invalid characters for your OS.

Get-ChildItem *.txt | Rename-Item -NewName {
    $firstLine = ($_ | Get-Content -TotalCount 1 -Encoding utf8).
        Normalize([Text.NormalizationForm]::FormD) -replace '[^a-z0-9 ]'

    '{0}.txt' -f $firstLine
}

Example:

$string = 'áÁéÉñÑ?'
$string.Normalize([Text.NormalizationForm]::FormD) -replace '[^a-z0-9 ?]'

# Outputs:
# aAeEnN?

Worth noting, default Get-Content encoding will be problematic in Windows PowerShell:

> Default Uses the encoding that corresponds to the system's active code page (usually ANSI).

Thus the need for -Encoding utf8. Newer PowerShell versions don't have such problem as they default to utf8NoBOM.

All you need to do is to add á and é to exclusion list of your replacement, ant they will be preserved:

Get-ChildItem *.txt | Rename-Item -NewName {
    ($_ | Get-Content -TotalCount 1 -Encoding UTF8) -replace '[^a-z0-9éá ]', '' -replace '.*', '$0.txt'
}

As for ? - it is not valid symbol for filename in windows, so I don't see a point there. But you always can do multiple replacement, and replace it with something allowed. Like so:

"asd we'wea?gke é or á? to b" -replace '[^a-z0-9éá ]', '' -replace '\?', '!!!!'

Santiago Squarzon's helpful answer shows you how to transform accented letters - such as é - to their unaccented form, such as e, causing them to be covered by the a-z regex range expression.

As for preserving accented characters as-is (which you state is acceptable too):

In lieu of a-z you can use \p{Ll}, which matches any Unicode lowercase letter therefore also accented ones (see the list of all Unicode categories).
By virtue of -replace being case-insensitive, uppercase letters are implicitly considered as well:

Get-ChildItem *.txt | Rename-Item -NewName {
  $firstLine = 
    ($_ | Get-Content -TotalCount 1 -Encoding utf8) -replace '[^\p{Ll}0-9 ]'
  '{0}.txt' -f $firstLine
}

^{Note: I'm using -Encoding utf8, as in the other answers, to read your file, which is only necessary in Windows PowerShell if your file happens to be UTF-8-encoded but without a BOM.}

A simplified example:

# -> 'aÄ éE 42'; that is, all letters and digits were preserved.
'a-Ä é/E 42' -replace '[^\p{Ll}0-9 ]'