Assuming a SPARQL query should filter on the length of labels:

SELECT ?label
FROM <http://mygraph.com>
WHERE
  {
    ?s ?p ?label
    FILTER(regex(str(?label), "^.{1,256}$") )
  }

ISQL uses '$ ' character as a prefix for macro names of its preprocessor. When '$ ' character is used in SPARQL query to be executed in ISQL, the character should be replaced with '$$ ' notation or an escape char + numeric code:

SQL> SPARQL
SELECT ?label
FROM <http://mygraph.com>
WHERE
  {
    ?s ?p ?label
    FILTER(REGEX(str(?label), "^.{1,256}$$") )
  }

Note also that the FILTER written in this way, finds ?label-s with length less than 256.

To achieve fast results, REGEX should be replaced with the bif:length function:

SQL> SPARQL
SELECT ?label
FROM <http://mygraph.com>
WHERE
  {
    ?s ?p ?label
    FILTER (bif:length(str(?label))<= 256)
  }

In this way the SPARQL query execution can work much faster if the interoperability is not required and ?label-s are numerous.