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 CXX, Regular Expression Functions (Perl-Compatible)
Reference XCIII, Microsoft SQL Server and Sybase Functions (PDO_DBLIB)
Reference XLIII, Firebird/Interbase Functions (PDO_FIREBIRD)
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.