Implement regexp-based thread ID parsing from recipient email

src/threads/id.ts

@@ -0,0 +1,22 @@
+import {config} from "../config.ts";
+
+function escapeRexString(rex: string): string {
+    const specials = ['.', '+', '(', ')', '[', ']', '^', '$']
+    for ( const char of specials ) {
+        rex = rex.replaceAll(char, `\\${char}`)
+    }
+    return rex
+}
+
+export function buildThreadAddressMatcher(): RegExp {
+    // We want a regular expression that only matches the thread address
+    // template specified by the config. We also want it to have a capture
+    // group for the thread ID.
+    // In the config, the thread ID is represented with the placeholder: %ID%
+
+    const idPrefix = escapeRexString(config.mail.threads.idPrefix)
+    const idCapture = `(${idPrefix}[^@]+)`  // first rule of email: don't be as strict as the RFC
+
+    const template = escapeRexString(config.mail.threads.template).replace('%ID%', idCapture)
+    return new RegExp(`^${template}$`)
+}