void setFilter(cdbfapiPlus* handle, char* expression);

Function setFilter() filters records by some condition.
The function does not change a file, it filters records in a memory.
To return the original state of a table - call unsetFilter().
By default case of letters is ignored. You can change the default behaviour with function caseSensitiveMode().
You can use the following operators in the expression:

Operation	Meaning	Example
<	Less	FIELD1<123
>	More	FIELD1>123
=	Equal for numbers, begin with for strings	FIELD1=123 FIELD2=john
==	Equal for numbers, exact equal for strings	FIELD2==john FIELD2==john lennon
<>	Do not equal	FIELD1<>123
<=	Less or equal	FIELD1<=123
>=	More or equal	FIELD1>=123
~	Contain a substring	FIELD2~lennon
^	Do not contain a substring	FIELD1^lennon

You can use & and | to unite expressions

void unsetFilter(cdbfapiPlus* handle);
void caseSensitiveMode(cdbfapiPlus* handle, BOOL sensitive);

{
    // your code
    setFilter(dbf, "FIRSTNAME=john");
    printf( "Reccount = %ld\n", recCount(dbf) );
    caseSensitiveMode(dbf, TRUE);
    setFilter(dbf, "FIRSTNAME=john");
    printf( "Reccount = %ld\n", recCount(dbf) );
    // your code
    unsetFilter(dbf);
}