Add scan_wikitext example

examples/advanced/scan_wikitext.py

@@ -0,0 +1,42 @@
+"""
+Showcases how to use `Lark.scan` to select a pattern from a larger text without having to parse all of it.
+
+Uses `requests` to fetch the current wikitext from `Python (Programming Language)` and uses a simple grammar
+to extract all wikitext templates used in the page.
+
+"""
+
+from collections import Counter
+from pprint import pprint
+
+import lark
+import requests
+
+page_name = "Python_(programming_language)"
+url = f"https://en.wikipedia.org/wiki/{page_name}?action=raw"
+
+wikitext = requests.get(url).text
+
+grammar = r"""
+template: "{{" TEXT ("|" argument)* "}}"
+text: (TEXT|template)+
+argument: /\w+(?==)/ "=" text -> named_argument
+        | text -> numbered_argument
+
+TEXT: / (?:[^{}|]
+      | \{(?!\{)
+      | \}(?!\})
+      )+/x
+"""
+parser = lark.Lark(grammar, parser='lalr', start='template')
+used_templates = Counter()
+inner_templates = 0
+for (start, end), res in parser.scan(wikitext):
+    for temp in res.find_data('template'):
+        if temp != res:
+            inner_templates += 1
+        used_templates[temp.children[0].value] += 1
+
+pprint(used_templates)
+print("Total templates used:", used_templates.total())
+print("Number of templates nested inside others:", inner_templates)