reftex-index-phrases-search-whole-words

reftex-index-phrases-search-whole-words is a variable defined in `reftex-vars.el'.
Its value is

t

Documentation:
Non-nil means phrases search will look for whole words, not subwords.
This works by requiring word boundaries at the beginning and end of
the search string. When the search phrase already has a non-word-char
at one of these points, no word boundary is required there.

You can customize this variable.