Add org-ql-plain-string-predicate and use it

README.org

@@ -74,6 +74,8 @@ These commands and functions are included:
      -  ~org-ql-select~ (function)
      -  ~org-ql-query~ (function)
 
+If user search bare string, the predicate ~org-ql-plain-string-predicate~ will be used by default.
+
 Feedback on these APIs is welcome.  Eventually, after being tested and polished, they will be considered stable. 
 
 # TODO: Find a way to get these examples included in the info manual.
@@ -378,6 +380,8 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 ** 0.4-pre
 
 *Added*
++  *Variable*
+     -  ~org-ql-plain-string-predicate~, which will be used when user search bare string.
 +  *Commands*
      -  ~helm-org-ql-views~, which shows one of ~org-ql-views~ selected with Helm.
      -  ~org-ql-search~ can search files in ~org-directory~; customization options are available in the ~org-ql-search~ group.

org-ql.el

@@ -109,6 +109,9 @@ the value returned by it at that node.")
   "Plist of predicates, their corresponding functions, and their docstrings.
 This list should not contain any duplicates.")
 
+(defvar org-ql-plain-string-predicate 'regexp
+  "Plain strings in queries are matched with this predicate.")
+
 ;;;; Customization
 
 (defgroup org-ql nil
@@ -521,8 +524,9 @@ Or, when possible, fix the problem."
 
 (defun org-ql--pre-process-query (query)
   "Return QUERY having been pre-processed.
-Replaces bare strings with (regexp) selectors, and appropriate
+Replaces bare strings with predicate set by
-`ts'-related selectors."
+`org-ql-plain-string-predicate' , and appropriate `ts'-related
+selectors."
   ;; This is unsophisticated, but it works.
   ;; TODO: Maybe query pre-processing should be done in one place,
   ;; rather than here and in --query-predicate.
@@ -538,7 +542,7 @@ Replaces bare strings with (regexp) selectors, and appropriate
                      (`(unless ,condition . ,clauses) `(unless ,(rec condition)
                                                          ,@(mapcar #'rec clauses)))
                      ;; TODO: Combine (regexp) when appropriate (i.e. inside an OR, not an AND).
-                     ((pred stringp) `(regexp ,element))
+                     ((pred stringp) `(,org-ql-plain-string-predicate ,element))
                      ;; Quote children queries so the user doesn't have to.
                      (`(children ,query) `(children ',query))
                      (`(children) '(children (lambda () t)))
@@ -1495,7 +1499,8 @@ Builds the PEG expression using predicates defined in
                             (-sort (-on #'> #'length)))))
       `(cl-defun org-ql--plain-query (input &optional (boolean 'and))
          "Return query parsed from plain query string INPUT.
-Multiple predicates are combined with BOOLEAN."
+Multiple predicates are combined with BOOLEAN.
+If no predicates are found, `org-ql-plain-string-predicate' will be used."
          (unless (s-blank-str? input)
            (let* ((query (peg-parse-string
                           ((query (+ term
@@ -1506,7 +1511,7 @@ Multiple predicates are combined with BOOLEAN."
                                      positive-term))
                            (positive-term (or (and predicate-with-args `(pred args -- (cons (intern pred) args)))
                                               (and predicate-without-args `(pred -- (list (intern pred))))
-                                              (and plain-string `(s -- (list 'regexp s)))))
+                                              (and plain-string `(s -- (list org-ql-plain-string-predicate s)))))
                            (plain-string (or quoted-arg unquoted-arg))
                            (predicate-with-args (substring predicate) ":" args)
                            (predicate-without-args (substring predicate) ":")