Search for files & file names using silver searcher

According to the man page of ag

   -G --file-search-regex PATTERN
          Only search files whose names match PATTERN.

You can use the -G option to perform searches on files matching a pattern.

So, to answer your question:

root@apache107:~/rpm-4.12.0.1# ag -G cpio.c size
rpm2cpio.c
21:    off_t payload_size;
73:    /* Retrieve payload size and compression type. */
76:     payload_size = headerGetNumber(h, RPMTAG_LONGARCHIVESIZE);

the above command searches for the word size in all files that matches the pattern cpio.c

Reference:
man page of ag version 0.28.0

Note 1:

If you are looking for a string in certain file types, say all C sources code, there is an undocumented feature in ag to help you quickly restrict searches to certain file types.

The commands below both look for foo in all php files:

find . -name \*.php -exec grep foo {}
ag --php foo

While find + grep looks for all .php files, the --php switch in the ag command actually looks for the following file extensions:

.php  .phpt  .php3  .php4  .php5  .phtml

You can use --cpp for C++ source files, --hh for .h files, --js for JavaScript etc etc. A full list can be found here

Note 2:

Keep in mind that when using language specific flags such as --php alongside the --file-search-regex option, then the --file-search-regex appears to be ignored at least per this example below

$ mkdir ag-test
$ cd ag-test
$ echo hello > blah.php
$ echo hello > mmkay.php
$ echo hello > special_something.txt
$ echo hello > special_also.php
$ echo nothing > nope.php
$ echo nothing > nada.php
$ echo nothing > special.php

# Only --php
$ ag --php  -l hello .|cat
special_also.php
mmkay.php
blah.php

# Only --file-search-regex
$ ag  --file-search-regex special  -l hello .|cat
special_also.php
special_something.txt

# Both
$ ag --php --file-search-regex special  -l hello .|cat
special_also.php
mmkay.php
blah.php