add regexp.Match to template funcs

Add a "Match" template func, which uses regexp.MatchString to match a
value against a regular expression pattern.

Fixes #125

Signed-off-by: Will Norris <will@tailscale.com>

golink.go

@@ -629,6 +629,12 @@ var expandFuncMap = texttemplate.FuncMap{
 	"PathEscape":  url.PathEscape,
 	"QueryEscape": url.QueryEscape,
 	"TrimSuffix":  strings.TrimSuffix,
+	"Match":       regexMatch,
+}
+
+func regexMatch(pattern string, s string) bool {
+	b, _ := regexp.MatchString(pattern, s)
+	return b
 }
 
 // expandLink returns the expanded long URL to redirect to, executing any

golink_test.go

@@ -417,6 +417,18 @@ func TestExpandLink(t *testing.T) {
 			remainder: "a/",
 			want:      "http://host.com/a",
 		},
+		{
+			name:      "template-with-match-func",
+			long:      `http://host.com/{{if Match "\\d+" .Path}}id/{{.Path}}{{else}}search/{{.Path}}{{end}}`,
+			remainder: "123",
+			want:      "http://host.com/id/123",
+		},
+		{
+			name:      "template-with-match-func2",
+			long:      `http://host.com/{{if Match "\\d+" .Path}}id/{{.Path}}{{else}}search/{{.Path}}{{end}}`,
+			remainder: "query",
+			want:      "http://host.com/search/query",
+		},
 		{
 			name:      "relative-link",
 			long:      `rel`,

tmpl/help.html

@@ -55,6 +55,8 @@ <h2 id="advanced">Advanced destination links</h2>
 <ul>
   <li><code>PathEscape</code> is the <a href="https://pkg.go.dev/net/url#PathEscape">url.PathEscape</a> function for escaping values inside a URL path.
   <li><code>QueryEscape</code> is the <a href="https://pkg.go.dev/net/url#QueryEscape">url.QueryEscape</a> function for escaping values inside a URL query.
+  <li><code>TrimSuffix</code> is the <a href="https://pkg.go.dev/strings#TrimSuffix">strings.TrimSuffix</a> function for removing a trailing suffix.
+  <li><code>Match</code> is the <a href="https://pkg.go.dev/regexp#MatchString">regexp.MatchString</a> function for matching a regular expression pattern.
 </ul>
 
 <p>