x-pixel-size-width-font-regexp

x-pixel-size-width-font-regexp is a variable defined in `C source code'.
Its value is

"gb2312\\|gbk\\|gb18030\\|jisx0208\\|ksc5601\\|cns11643\\|big5"

Documentation:
Regexp matching a font name whose width is the same as `PIXEL_SIZE'.

Since Emacs gets width of a font matching with this regexp from
PIXEL_SIZE field of the name, font finding mechanism gets faster for
such a font. This is especially effective for such large fonts as
Chinese, Japanese, and Korean.