STACK

$S_{str}\ S_{seek}\ --\ S_{post}\ S_{match}\ B_{true}$
$S_{str}\ S_{seek}\ --\ S_{str}\ B_{false}$

DESCRIPTION

Red book operator determines whether the string seek matches the initial substring of str.
If it matches, anchorsearch splits string into two segments
match, the portion of str that matches seek, and post, the remainder of str
and returns the string objects post and match followed by the boolean value true.
Otherwise, it returns the original str followed by false. anchorsearch is a special case of the search operator.

ORIGIN

Core code registered in systemdict.