Where is less search pattern reference?

less's man page says:

   /pattern
          Search forward in the file for the N-th line containing
          the pattern.  N defaults to 1.  The pattern is a regular
          expression, as recognized by the regular expression library
          supplied by your system.

so the accepted syntax may depend on your system. Off-hand, it seems to accept extended regular expressions on my Debian system, see regex(7), and Why does my regular expression work in X but not in Y?

\d is from Perl, and isn't supported by all regex engines. Use [0-9] or [[:digit:]] to match digits. (Their exact behaviour may depend on the locale.)

The expressions supported by less are documented in the re_format(7) manual (man 7 re_format). That manual describes both the extended regular expressions and the basic regular expressions available on your system. The less utility understands extended regular expressions.

To match a digit, you would use [0-9] or [[:digit:]] (there's a slight difference as the former depends on the current locale). The \d pattern is a Perl-like regular expression (PCRE), not supported by less.