parse: Fix bug in handling of link destinations

This was a very silly bug: a missing ^ at the start of a regular
expression that almost always failed to match despite scanning the
entire input most of the time, when it was called. The result was some
very strange behavior when it did happen to match.

fixes #4

Author: lexi-lambda

commonmark-lib/commonmark/private/parse/common.rkt

@@ -122,7 +122,7 @@
 
 ;; <https://spec.commonmark.org/0.30/#link-destination>
 (define (try-peek-link-destination in [start-pos 0])
-  (match (regexp-match-peek #px"<([^\r\n]*?)(?<!\\\\)>" in start-pos)
+  (match (regexp-match-peek #px"^<([^\r\n]*?)(?<!\\\\)>" in start-pos)
     ; First, the simple case: a destination enclosed in <angle brackets>.
     [(list peeked-bytes dest-bytes)
      (list (+ start-pos (bytes-length peeked-bytes))

commonmark-test/tests/commonmark/parse/gh4.rkt

@@ -0,0 +1,53 @@
+#lang racket/base
+
+;; Regression test for <https://github.com/lexi-lambda/racket-commonmark/issues/4>.
+
+(require commonmark/struct
+         rackunit
+         "../test-util.rkt")
+
+(check-equal? (md "[a link](http://example.com).</span>")
+              (document
+               (list
+                (paragraph
+                 (list (link "a link" "http://example.com" #f) "." (html "</span>"))))
+               '()))
+
+(check-equal? (md "[a link][1].</span>"
+                  ""
+                  "[1]: http://example.com")
+              (document
+               (list
+                (paragraph
+                 (list (link "a link" "http://example.com" #f) "." (html "</span>"))))
+               '()))
+
+(check-equal? (md "This <span>is _emphasis_ and [a link](http://example.com).</span>")
+              (document
+               (list
+                (paragraph
+                 (list "This " (html "<span>") "is " (italic "emphasis") " and " (link "a link" "http://example.com" #f) "." (html "</span>"))))
+               '()))
+
+(check-equal? (md "This <span>is _emphasis_ and [a link][1].</span>"
+                  ""
+                  "[1]: http://example.com")
+              (document
+               (list
+                (paragraph
+                 (list "This " (html "<span>") "is " (italic "emphasis") " and " (link "a link" "http://example.com" #f) "." (html "</span>"))))
+               '()))
+
+(check-equal? (md "This <span>is _emphasis_ and [a link](http://example.com).")
+              (document
+               (list
+                (paragraph
+                 (list "This " (html "<span>") "is " (italic "emphasis") " and " (link "a link" "http://example.com" #f) ".")))
+               '()))
+
+(check-equal? (md "This <span>is _emphasis_ and [a link](http://example.com).</div>")
+              (document
+               (list
+                (paragraph
+                 (list "This " (html "<span>") "is " (italic "emphasis") " and " (link "a link" "http://example.com" #f) "." (html "</div>"))))
+               '()))