Add anchor extraction function

`anchor.extract(string, anchor)` will find and extract a zettel anchor
from any string passed in.

It can be given an optional anchor regex which will supersede any regex
set in the plugin options.

README.md

@@ -8,9 +8,10 @@ next up:
 [ ] fix link following:
   * [x] empty space (e.g. in link text, or link itself) disrupts link regex search
   * [x] line-end following breaks if cursor is in the MIDDLE of the link
-  * [ ] extract anchor from link/string (anchor.lua)
+  * [x] extract anchor from link/string (anchor.lua)
+    * [x] probably stop hardcoding anchor regex, make an option
+    * [ ] implement custom anchor creation function to go with custom regex
   * [ ] opening zettel should use generated link table for full filename anchor search
-    * [ ] probably stop hardcoding anchor regex, make an option
   * [ ] implement fallback to filename
 
 ## TODO: needed functionality

lua/zettelkasten/anchor.lua

@@ -26,4 +26,15 @@ function A.prepend(anchor, text)
     return text
 end
 
+-- Returns anchor contents if an anchor is contained in the input string.
+-- It takes an optional regex parameter with which the plugin option
+-- of which anchor to look for can be overwritten.
+--
+-- If multiple anchors are contained, returns the contents of the first
+-- one encountered.
+function A.extract(input, regex)
+    regex = regex or o.anchor().regex
+    return input:match(regex)
+end
+
 return A

lua/zettelkasten/anchor_spec.lua

@@ -39,3 +39,25 @@ describe("prepend", function()
         assert.same("1910291645---arated", A.prepend("1910291645", "arated"))
     end)
 end)
+
+describe("extract", function()
+    it("should get the default anchor from a string of text", function()
+        assert.same("2010261208", A.extract(
+                        "/home/office/docs/2010261208 we are the champions.md"))
+    end)
+    it("should return nil when default anchor not contained", function()
+        assert.same(nil, A.extract(
+                        "/home/office/docs/10261208 we are the champions.md"))
+    end)
+    it("should use the anchor set in options", function()
+        vim.g.zettel_anchor_regex = '[%u][%l][%d][%d][%d][%d]'
+        assert.same("Fa1984", A.extract(
+                        "/home/office/docs/Fa1984_we are the champions.md"))
+    end)
+    it("should use the anchor regex argument if one is passed", function()
+        assert.same("bO133T",
+                    A.extract(
+                        "/home/office/docs/bO133T-we are the champions.md",
+                        "[%l][%u][%d][%d][%d][%u]"))
+    end)
+end)