Match whole word from partial-word search string

Could you help me to create a pattern, which matches whole words, containing a specific part? For example, if I have a text string Perform a regular expression match and if I search for express, it should give me expression, if I search for form, it should give me Perform and so on.

preg_match('/\b(express\w+)\b/', $string, $matches); // matches expression
preg_match('/\b(\w*form\w*)\b/', $string, $matches); // matches perform,
                                                     // formation, unformatted

Where:

• \b is a word boundary
• \w+ is one or more "word" character^*
• \w* is zero or more "word" characters

See the manual on escape sequences for PCRE.

^{* Note: although not really a "word character", the underscore _ is also included int the character class \w.}