How do I write a Clojure macro to create a regular expression from a String?

Question

I'm creating a convenience macro. Part of the convenience is that a regular expression can be specified with just a String, rather than the #"re" notation.

The one part I can't figure out is how to get the macro to take the String and rewrite it as a Clojure regex (e.g., produce the #"re" notation). I think it's a syntax / escaping problem.

My first naive attempt (pretending I only want the String-to-regex part):

(defmacro mymac [mystr] `#~mystr)

Is it even possible to do what I'm trying to do? Or, is there an actual function to take a String and produce a regex, instead of using the # reader macro?

Or should I just drop into Java and use java.util.regex.Pattern?

sinemetu1 · Accepted Answer · 2012-09-16 04:00:20Z

30

There is a function for it: re-pattern

user=> (re-pattern "\\d+")
#"\d+"

edited Sep 16, 2012 at 4:00

sinemetu1

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answered Aug 6, 2010 at 5:14

j-g-faustus

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kotarak · Accepted Answer · 2010-08-06 09:39:47Z

8

To explain a bit more:

#"" is a reader macro. It is resolved at read time by the reader. So there is no way to create a macro which expands into a reader macro, because the read phase is long gone. A macro returns the actual data structure representing the expanded code, not a string which is parsed again like eg. #define works in C.

j-g-faustus' answer is the Right Way(tm) to go.

answered Aug 6, 2010 at 9:39

kotarak

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1 Comment

dirtyvagabond Over a year ago

Thanks kotarak. I should have realized why the reader life cycle prevents me from doing what I was trying to do. Thanks!

Sean · Accepted Answer · 2010-08-06 04:49:32Z

0

I may be misunderstanding the question, but doesn't this do what you want?

user=> (. java.util.regex.Pattern compile "mystr")
#"mystr"

answered Aug 6, 2010 at 4:49

Sean

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1 Comment

dirtyvagabond Over a year ago

yes it does exactly what i want. but you have to admit, not as pretty as j-g-faustus's (re-pattern "mystr")

willscripted · Accepted Answer · 2015-07-09 12:21:09Z

0

To match a string verbatim, ignoring special characters:

(defn to-regex [some-string]
  (re-pattern (java.util.regex.Pattern/quote some-string)))

Then ... will only match ..., not aaa or any other three letter combination.

answered Jul 9, 2015 at 12:21

willscripted

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