Postgres 10

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CREATE TABLE term_size_ref
             (term_size text);
INSERT INTO term_size_ref
            (term_size)
            VALUES ('A0.01'),
                   ('B+100'),
                   ('2.1'),
                   ('S'),
                   ('B0.0001'),
                   ('A0.001'),
                   ('B-7'),
                   ('RH'),
                   ('RH99'),
                   ('.1'),
                   ('-1'),
                   ('+4'),
                   ('2');
13 rows affected

SELECT *
       FROM term_size_ref
       ORDER BY regexp_replace(term_size,
                               '[\d.\-+]+$',
                               '') ASC,
                nullif(regexp_replace(term_size,
                                     '^[^\d.\-+]+',
                                     ''),
                       '')::decimal ASC
                                    NULLS FIRST;


term_size


-1

.1

2

2.1

+4

A0.001

A0.01

B-7

B0.0001

B+100

RH

RH99

S