Wildcard alias for Elastic index

You're almost there. It's the other way around, i.e. POST /<index>/_alias/<alias>

POST /transactions-*-internal/_alias/transactions-internal

UPDATE:

If you want the other way around, then you can use the following (note that an alias name cannot contain wildcard characters):

POST /_aliases{    "actions" : [        { "add" : { "index" : "transactions-internal", "alias" : "transactions-a-internal" } },        { "add" : { "index" : "transactions-internal", "alias" : "transactions-b-internal" } },        { "add" : { "index" : "transactions-internal", "alias" : "transactions-c-internal" } }    ]}

Not quite sure if this is applicable to your situation but if you're starting from scratch a possible solution might be to use a index template.

PUT _index_template/transactions-internal{  "priority": 1,  "template": {    "aliases": {      "transactions-internal": {}    }  },  "index_patterns": [    "transactions-*-internal"  ],  "composed_of": []}

As I'm quite new to elastic I don't know if this template can be applied to an existing index.But this approach will work for new indizes in v 7.12.1