rcsb.org supports searching for sequence motifs defined by regex (https://en.wikipedia.org/wiki/Regular_expression). With regex you get access to operators such as '^' (start of line) or '$' (end of line). You can e.g. search for N-terminal His-Tags with the following query:

https://www.rcsb.org/search?request=%7B%22query%22%3A%7B%22type%22%3A%22group%22%2C%22logical_operator%22%3A%22and%22%2C%22nodes%22%3A%5B%7B%22type%22%3A%22terminal%22%2C%22service%22%3A%22seqmotif%22%2C%22parameters%22%3A%7B%22value%22%3A%22%5EHHHHHH%22%2C%22pattern_type%22%3A%22regex%22%2C%22target%22%3A%22pdb_protein_sequence%22%7D%7D%5D%7D%2C%22return_type%22%3A%22polymer_entity%22%2C%22request_info%22%3A%7B%22query_id%22%3A%22258c1148bff15149870108075d0dd677%22%7D%2C%22request_options%22%3A%7B%22pager%22%3A%7B%22start%22%3A0%2C%22rows%22%3A25%7D%2C%22scoring_strategy%22%3A%22combined%22%2C%22sort%22%3A%5B%7B%22sort_by%22%3A%22score%22%2C%22direction%22%3A%22desc%22%7D%5D%7D%7D

Documentation can be found here: https://www.rcsb.org/docs/search-and-browse/advanced-search/sequence-motif-search