These are not actual extensions. They are part of the PHP core and cannot be left out of a PHP binary with compilation options.
These extensions are bundled with PHP.
Reference VII, BCMath Arbitrary Precision Mathematics Functions
Reference XXIV, Database (dbm-style) Abstraction Layer Functions
Reference CXI, Regular Expression Functions (Perl-Compatible)
Reference LXXXIV, Microsoft SQL Server and Sybase Functions (PDO_DBLIB)
Reference CXXXIV, Semaphore, Shared Memory and IPC Functions
In order to compile these extensions, external libraries will be needed.
These extensions are available from PECL. More PECL extensions exist but they are not documented in the PHP manual yet.