7. The document processing apparatus according to claim 1 , further comprising: a filter storing unit configured to store a plurality of filters associated with application programs, respectively; and a filter selecting unit configured to select a filter associated with the first application program from the filter storing unit, wherein the character string extracting unit is configured to detect the one or more character strings using the filter selected by the filter selecting unit.
7. The document processing apparatus according to claim 1 , further comprising: a filter storing unit configured to store a plurality of filters associated with application programs, respectively; and a filter selecting unit configured to select a filter associated with the first application program from the filter storing unit, wherein the character string extracting unit is configured to detect the one or more character strings using the filter selected by the filter selecting unit. 9. The document processing apparatus according to claim 7 , further comprising: a user setting unit configure to display one or more filters stored by the filter storing unit as a candidate for selection to the user to enable the user to select at least one of the one or more filters to be used in the character string extracting unit in preference to a relationship between an application program and a filter determined by the filter storing unit.