@Jacccs
An example or detailed description of yoru use case may be helpful in providing the bets guidance for you. While the NiFi Expression Language (NEL) function anyMatchingAttribute expects a java regular expression that searches and returns values for multiple FlowFile attributes, that does not appear to be what you need??? Your attribute "attributeToSearch" implies only a single specific FlowFile attribute is desire to be checked if it contains some "${value}". If this is correct, you would be able to use the following NEL:
${literal("${${attributeToSearch}}"):contains('${value}')}
For above NEL, let's assume a FlowFile with attribute "attributeToSearch" set to "username". A FlowFile attribute "username" set to "admin-matt". A FlowFile attribute "value" set to "admin".
The result of above NEL statement would be true
${$attributeToSearch}} would first resolve to ${username} which would then resolve to "admin-matt". That "admin-matt" string would be then passed to the NEL contains function which will to check to see if that string contains the string "admin" within it. The result is a boolean "true" or "false".
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Thank you,
Matt