Use a Power Automate Flow to Scrub File Names of Unwanted Characters

Last year, my team rolled out a Power App Portal (Power Pages) to allow customers to submit requests with attachments. The attachments are stored in Azure Blob Storage, and we use Cloud Mersive to virus scan the submitted attachments. Not to get too deep into the weeds, the process flows like this:
Attachment is uploaded –> lands in the Dataverse Note (annotation) table –> then is shipped to blob storage

Now the problem: users can name a file whatever they like and upload them. This quickly became an issue due to Flow not always being able to find the blobs associated with the request if the filename contained some special characters. Example: MyTrademark®.pdf

There are some great examples online for replacing special characters with a space or another supported character, but I wanted to take a different approach that seemed a lot more efficient to me. Where my example differs is the use of the Filter Array Flow action to only check the characters of the filename, as opposed to looping through each letter of the alphabet and comparing it to each letter in the filename.

Here’s the completed Flow, but I’ll dig into each step in this post.

The Compose Chars action holds the array of characters I will use to validate the characters in the supplied filename. This can be shortened if the input filename is to be set to uppercase or lowercase; only one set of the alphabet is needed.


Compose Org Filename: string('my super 123 longer $%^&^ file /// name ^^^ with junk in it.xlsx')
Compose Split Extension: last(split(outputs('Compose_Org_Filename'), '.'))
Compose Concat Extension: concat('.', outputs('Compose_Split_Extension'))
Compose Get Filename: split(outputs('Compose_Org_Filename'), outputs('Compose_Concat_Extension'))[0]

The point of the Apply to each loop is to iterate over each item in the filename. Note: I’m using a Chunk function to break apart the filename. I first tried using a Spilt function, but there would be no end to what the delimiter might be.

Apply to each: chunk(outputs('Compose_Get_Filename'),1)

Filter array Chars: From: Compose Chars
char is equal to Current item
Here is the advanced view of the action:
@equals(item()?['char'], items('Apply_to_each'))
If you think of it like a SQL statement, it would be:
Select * from Compose Chars Where Char = Current item
The filter checks if the current item in the apply to each loop is in the Compose Chars array.

Condition: empty(body('Filter_array_Chars')) is equal to true
If the current item is not in the array, skip it (yes), else start building the filename (no)

This hack is needed due to not being able to set a Flow action equal to itself. Think of it like a programmatic iteration. i++ or i = i + 1

Compose Temp is a placeholder for the varNameBuilder variable.
Set variable Name Builder: concat(outputs('Compose_Temp'),items('Apply_to_each'))

Compose Clean Filename: concat(variables('varNameBuilder'),outputs('Compose_Concat_Extension'))

Copy of the Flow can be downloaded here:

Things to consider:
Empty filename –
What if the filename is nothing but special / unwanted characters? At the end of the Flow, you’d want to use a Length function to check varNameBuilder to see if it’s greater than X.
example: !@#@$#$%%^&.pdf
The result from the Flow would be .pdf, and updating the filename would fail. To my knowledge, you can’t name a file like that, but you get the point.

Also, I’ve had users upload files with non-English characters, so there is a viable chance that someone, at some point, might upload a file like this: 我喜欢炸玉米饼.pdf

Making the Flow available to other flows –
If the Flow is created in a solution, it could be used more like a function, and other Flows in the solution could reference it. This would be a great example of a reusable Child Flow.