# Need a bit of help with an instaparse grammar
**URL:** https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965
**Category:** How to?
**Created:** [November 23, 2017, 8:26pm UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965 "2017-11-23T20:26:01Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@JeremS](https://clojureverse.org/u/JeremS)
#### Post date: [November 23, 2017, 8:26pm UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965/1 "2017-11-23T20:26:01Z")
Hey guys,
I am playing with instaparse and I have a problem contructucting a grammar.
Here is what I am going for
```clojure
(insta/defparser ex7
"
doc = (text | tag)*
text = #'[^@]*'
tag = '@' #'[a-z]*' inner-text*
inner-text = '{' #'[^}]*' '}'
")
(ex7 "some text @toto{inner text}")
```
The problem is the parser when parsing a `tag` rule won’t consider the `inner-text` rule giving me the parse
```auto
[:doc [:text "some text "] [:tag "@" "toto"] [:text "{inner text}"]]
```
instead of the desired :
```auto
[:doc [:text "some text "] [:tag "@" "toto" [:inner-text "{" "inner text" "}"]]]
```
Any idea how I can modify the grammar so that the parser would consider the `inner-text rule` before going back to the `text` one ?
---
### Author:  [@floop](https://clojureverse.org/u/floop)
#### Post date: [November 23, 2017, 11:29pm UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965/2 "2017-11-23T23:29:40Z")
**ordered choice operator**
Instead of alternation `|`:
```auto
doc = (text | tag)*
```
try the [ordered choice](https://github.com/Engelberg/instaparse#ordered-choice) operator `/`:
```auto
doc = (tag / text)*
```
**debugging**
As you may know, `insta/parses` helps debug ambiguous grammars, by showing all parses.
```clojure
(->> (insta/parses ex7 "some text @toto{inner text}")
clojure.pprint/pprint)
```
**ensuring tags, even if empty**
I’d tend to ensure an `:inner-text`, even if empty, just to keep things regular:
```clojure
tag = '@' #'[a-z]*' inner-text
inner-text = ( '{' #'[^}]*' '}' )*
```
---
### Author:  [@plexus](https://clojureverse.org/u/plexus)
#### Post date: [November 24, 2017, 11:13am UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965/3 "2017-11-24T11:13:52Z")
Hi @JeremS, if this answer solved your problem then you can mark it as “resolved” by checking the little checkmark underneath @floop’s post! That way others know that your question has been answered and you don’t need more help right now.
---
### Author:  [@JeremS](https://clojureverse.org/u/JeremS)
#### Post date: [November 24, 2017, 12:49pm UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965/4 "2017-11-24T12:49:13Z")
Hey guys thank you for the reply. The ordered choice operator works !
As a conclusion to this thread, here is some code using @floop solution and another suggested to me by Alex Engelberg on the instaparse channel of the clojure slack.
Here is @floop’s solution using PEG’s ordered choice operator in the `doc` rule:
```auto
(insta/defparser ex-with-choice
"
doc = (tag / text)*
text = #'[^@]*'
tag = <'@'> #'[a-z]\\w*' text-block*
text-block = <'{'> inner-text <'}'>
= #'[^}]*'
")
```
Alex’s solution uses PEG’s negative lookahead operator:
```auto
(insta/defparser ex-with-lookahead
"
doc = (text | tag)*
text = #'[^@]*'
tag = <'@'> #'[a-z]\\w*' text-block* !text-block
text-block = <'{'> inner-text <'}'>
= #'[^}]*'
")
```
The solutions seem to yield the same results
```auto
(= (ex-with-choice "some text @tag1{inner text}{inner text 2} some text 2 @tag2 other text")
(ex-with-lookahead "some text @tag1{inner text}{inner text 2} some text 2 @tag2 other text"))
;=> true
```
Which is:
```auto
(ex-with-choice "some text @tag1{inner text}{inner text 2} some text 2 @tag2 other text")
;=>
[:doc
[:text "some text "]
[:tag "tag1" [:text-block "inner text"] [:text-block "inner text 2"]]
[:text " some text 2 "]
[:tag "tag2"]
[:text " other text"]]
```
The PEG extensions in instaparse are very useful indeed.
Thx again. Cheers,
---
### Author:  [@system](https://clojureverse.org/u/system)
#### Post date: [October 5, 2018, 12:50pm UTC](https://clojureverse.org/t/need-a-bit-of-help-with-an-instaparse-grammar/965/5 "2018-10-05T12:50:41Z")