sub

/*
 *  call-seq:
 *     sub(pattern, replacement)   => $_
 *     sub(pattern) { block }      => $_
 *  
 *  Equivalent to <code>$_.sub(<i>args</i>)</code>, except that
 *  <code>$_</code> will be updated if substitution occurs.
 */

static VALUE
rb_f_sub(argc, argv)
    int argc;
    VALUE *argv;
{
    VALUE str = rb_str_dup(uscore_get());

    if (NIL_P(rb_str_sub_bang(argc, argv, str)))
        return str;
    rb_lastline_set(str);
    return str;
}