The regular expression search

The re-search-forward function searches for the end of the sentence, that is, for the pattern defined by the sentence-end regular expression. If the pattern is found—if the end of the sentence is found—then the re-search-forward function does two things:

1. The re-search-forward function carries out a side effect, which is to move point to the end of the occurrence found.
2. The re-search-forward function returns a value of true. This is the value received by the if, and means that the search was successful.

The side effect, the movement of point, is completed before the if function is handed the value returned by the successful conclusion of the search.

When the if function receives the value of true from a successful call to re-search-forward, the if evaluates the then-part, which is the expression (skip-chars-backward " \t\n"). This expression moves backwards over any blank spaces, tabs or carriage returns until a printed character is found and then leaves point after the character. Since point has already been moved to the end of the pattern that marks the end of the sentence, this action leaves point right after the closing printed character of the sentence, which is usually a period.

On the other hand, if the re-search-forward function fails to find a pattern marking the end of the sentence, the function returns false. The false then causes the if to evaluate its third argument, which is (goto-char par-end): it moves point to the end of the paragraph.

(And if the text is in a form or equivalent, and point may not move fully, then the constrain-to-field function comes into play.)

Regular expression searches are exceptionally useful and the pattern illustrated by re-search-forward, in which the search is the test of an if expression, is handy. You will see or write code incorporating this pattern often.