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35.3.3.3 Defining new rx forms

The rx notation can be extended by defining new symbols and parameterized forms in terms of other rx expressions. This is handy for sharing parts between several regexps, and for making complex ones easier to build and understand by putting them together from smaller pieces.

For example, you could define name to mean (one-or-more letter), and (quoted x) to mean (seq ?' x ?') for any x. These forms could then be used in rx expressions like any other: (rx (quoted name)) would match a nonempty sequence of letters inside single quotes.

The Lisp macros below provide different ways of binding names to definitions. Common to all of them are the following rules:

• Built-in rx forms, like digit and group, cannot be redefined.
• The definitions live in a name space of their own, separate from that of Lisp variables. There is thus no need to attach a suffix like -regexp to names; they cannot collide with anything else.
• Definitions cannot refer to themselves recursively, directly or indirectly. If you find yourself needing this, you want a parser, not a regular expression.
• Definitions are only ever expanded in calls to rx or rx-to-string, not merely by their presence in definition macros. This means that the order of definitions doesn’t matter, even when they refer to each other, and that syntax errors only show up when they are used, not when they are defined.
• User-defined forms are allowed wherever arbitrary rx expressions are expected; for example, in the body of a zero-or-one form, but not inside any or category forms. They are also allowed inside not and intersection forms.

Macro: rx-define name [arglist] rx-form ¶

Define name globally in all subsequent calls to rx and rx-to-string. If arglist is absent, then name is defined as a plain symbol to be replaced with rx-form. Example:

(rx-define haskell-comment (seq "--" (zero-or-more nonl)))
(rx haskell-comment)
     ⇒ "--.*"

If arglist is present, it must be a list of zero or more argument names, and name is then defined as a parameterized form. When used in an rx expression as (name arg…), each arg will replace the corresponding argument name inside rx-form.

arglist may end in &rest and one final argument name, denoting a rest parameter. The rest parameter will expand to all extra actual argument values not matched by any other parameter in arglist, spliced into rx-form where it occurs. Example:

(rx-define moan (x y &rest r) (seq x (one-or-more y) r "!"))
(rx (moan "MOO" "A" "MEE" "OW"))
     ⇒ "MOOA+MEEOW!"

Since the definition is global, it is recommended to give name a package prefix to avoid name clashes with definitions elsewhere, as is usual when naming non-local variables and functions.

Forms defined this way only perform simple template substitution. For arbitrary computations, use them together with the rx forms eval, regexp or literal. Example:

(defun n-tuple-rx (n element)
  `(seq "<"
        (group-n 1 ,element)
        ,@(mapcar (lambda (i) `(seq ?, (group-n ,i ,element)))
                  (number-sequence 2 n))
        ">"))
(rx-define n-tuple (n element) (eval (n-tuple-rx n 'element)))
(rx (n-tuple 3 (+ (in "0-9"))))
  ⇒ "<\\(?1:[0-9]+\\),\\(?2:[0-9]+\\),\\(?3:[0-9]+\\)>"

Macro: rx-let (bindings…) body… ¶

Make the rx definitions in bindings available locally for rx macro invocations in body, which is then evaluated.

Each element of bindings is on the form (name [arglist] rx-form), where the parts have the same meaning as in rx-define above. Example:

(rx-let ((comma-separated (item) (seq item (0+ "," item)))
         (number (1+ digit))
         (numbers (comma-separated number)))
  (re-search-forward (rx "(" numbers ")")))

The definitions are only available during the macro-expansion of body, and are thus not present during execution of compiled code.

rx-let can be used not only inside a function, but also at top level to include global variable and function definitions that need to share a common set of rx forms. Since the names are local inside body, there is no need for any package prefixes. Example:

(rx-let ((phone-number (seq (opt ?+) (1+ (any digit ?-)))))
  (defun find-next-phone-number ()
    (re-search-forward (rx phone-number)))
  (defun phone-number-p (string)
    (string-match-p (rx bos phone-number eos) string)))

The scope of the rx-let bindings is lexical, which means that they are not visible outside body itself, even in functions called from body.

Macro: rx-let-eval bindings body… ¶

Evaluate bindings to a list of bindings as in rx-let, and evaluate body with those bindings in effect for calls to rx-to-string.

This macro is similar to rx-let, except that the bindings argument is evaluated (and thus needs to be quoted if it is a list literal), and the definitions are substituted at run time, which is required for rx-to-string to work. Example:

(rx-let-eval
    '((ponder (x) (seq "Where have all the " x " gone?")))
  (looking-at (rx-to-string
               '(ponder (or "flowers" "young girls"
                            "left socks")))))

Another difference from rx-let is that the bindings are dynamically scoped, and thus also available in functions called from body. However, they are not visible inside functions defined in body.