Regex to validate Android Package Name

android_pkg_name_validate.js

var pattern = /^[a-z][a-z0-9_]*(\.[a-z0-9_]+)+[0-9a-z_]$/i;
 
[
  "me.unfollowers.droid",
  "me_.unfollowers.droid",
  "me._unfollowers.droid",
  "me.unfo11llowers.droid",
  "me11.unfollowers.droid",
  "m11e.unfollowers.droid",
  "1me.unfollowers.droid",
  "me.unfollowers23.droid",
  "me.unfollowers.droid23d",
  "me.unfollowers_.droid",
  "me.unfollowers._droid",
  "me.unfollowers_._droid",
  "me.unfollowers.droid_",
  "me.unfollowers.droid32",
  "me.unfollowers.droid/",
  "me:.unfollowers.droid",
  ":me.unfollowers.droid",
  "me.unfollowers.dro;id",
  "me.unfollowe^rs.droid",
  "me.unfollowers.droid.",
  "me.unfollowers..droid",
  "me.unfollowers.droid._",
  "me.unfollowers.11212",
  "me.1.unfollowers.11212",
  "me..unfollowers.11212",
  "abc",
  "abc.",
  ".abc"
].forEach(function(pkg) {
    console.log('[' + (pattern.test(pkg) ? '✔' : '✘') + ']\t' + pkg);
});

result.md

Result

[✔] me.unfollowers.droid
[✔] me_.unfollowers.droid
[✔] me._unfollowers.droid
[✔] me.unfo11llowers.droid
[✔] me11.unfollowers.droid
[✔] m11e.unfollowers.droid
[✗] 1me.unfollowers.droid
[✔] me.unfollowers23.droid
[✔] me.unfollowers.droid23d
[✔] me.unfollowers_.droid
[✔] me.unfollowers._droid
[✔] me.unfollowers_._droid
[✔] me.unfollowers.droid_
[✔] me.unfollowers.droid32
[✗] me.unfollowers.droid/
[✗] me:.unfollowers.droid
[✗] :me.unfollowers.droid
[✗] me.unfollowers.dro;id
[✗] me.unfollowe^rs.droid
[✗] me.unfollowers.droid.
[✗] me.unfollowers..droid
[✗] me.unfollowers.droid._
[✔] me.unfollowers.11212
[✔] me.1.unfollowers.11212
[✗] me..unfollowers.11212
[✗] abc
[✗] abc.
[✗] .abc

If you want to be truly pedantic about it, then all of the Java Package Name rules need to be included, like no Java reserved words allowed. It makes for an epicly large regex:

https://stackoverflow.com/a/39331217

(?!^abstract$|^abstract\..*|.*\.abstract\..*|.*\.abstract$|^assert$|^assert\..*|.*\.assert\..*|.*\.assert$|^boolean$|^boolean\..*|.*\.boolean\..*|.*\.boolean$|^break$|^break\..*|.*\.break\..*|.*\.break$|^byte$|^byte\..*|.*\.byte\..*|.*\.byte$|^case$|^case\..*|.*\.case\..*|.*\.case$|^catch$|^catch\..*|.*\.catch\..*|.*\.catch$|^char$|^char\..*|.*\.char\..*|.*\.char$|^class$|^class\..*|.*\.class\..*|.*\.class$|^const$|^const\..*|.*\.const\..*|.*\.const$|^continue$|^continue\..*|.*\.continue\..*|.*\.continue$|^default$|^default\..*|.*\.default\..*|.*\.default$|^do$|^do\..*|.*\.do\..*|.*\.do$|^double$|^double\..*|.*\.double\..*|.*\.double$|^else$|^else\..*|.*\.else\..*|.*\.else$|^enum$|^enum\..*|.*\.enum\..*|.*\.enum$|^extends$|^extends\..*|.*\.extends\..*|.*\.extends$|^final$|^final\..*|.*\.final\..*|.*\.final$|^finally$|^finally\..*|.*\.finally\..*|.*\.finally$|^float$|^float\..*|.*\.float\..*|.*\.float$|^for$|^for\..*|.*\.for\..*|.*\.for$|^goto$|^goto\..*|.*\.goto\..*|.*\.goto$|^if$|^if\..*|.*\.if\..*|.*\.if$|^implements$|^implements\..*|.*\.implements\..*|.*\.implements$|^import$|^import\..*|.*\.import\..*|.*\.import$|^instanceof$|^instanceof\..*|.*\.instanceof\..*|.*\.instanceof$|^int$|^int\..*|.*\.int\..*|.*\.int$|^interface$|^interface\..*|.*\.interface\..*|.*\.interface$|^long$|^long\..*|.*\.long\..*|.*\.long$|^native$|^native\..*|.*\.native\..*|.*\.native$|^new$|^new\..*|.*\.new\..*|.*\.new$|^package$|^package\..*|.*\.package\..*|.*\.package$|^private$|^private\..*|.*\.private\..*|.*\.private$|^protected$|^protected\..*|.*\.protected\..*|.*\.protected$|^public$|^public\..*|.*\.public\..*|.*\.public$|^return$|^return\..*|.*\.return\..*|.*\.return$|^short$|^short\..*|.*\.short\..*|.*\.short$|^static$|^static\..*|.*\.static\..*|.*\.static$|^strictfp$|^strictfp\..*|.*\.strictfp\..*|.*\.strictfp$|^super$|^super\..*|.*\.super\..*|.*\.super$|^switch$|^switch\..*|.*\.switch\..*|.*\.switch$|^synchronized$|^synchronized\..*|.*\.synchronized\..*|.*\.synchronized$|^this$|^this\..*|.*\.this\..*|.*\.this$|^throw$|^throw\..*|.*\.throw\..*|.*\.throw$|^throws$|^throws\..*|.*\.throws\..*|.*\.throws$|^transient$|^transient\..*|.*\.transient\..*|.*\.transient$|^try$|^try\..*|.*\.try\..*|.*\.try$|^void$|^void\..*|.*\.void\..*|.*\.void$|^volatile$|^volatile\..*|.*\.volatile\..*|.*\.volatile$|^while$|^while\..*|.*\.while\..*|.*\.while$)(^(?:[a-z_]+(?:\d*[a-zA-Z_]*)*)(?:\.[a-z_]+(?:\d*[a-zA-Z_]*)*)*$)

This will include some invalid Application IDs. There must be at least two segment separated by ., and the first letter of each segment cannot be upper case or a number. https://developer.android.com/studio/build/application-id Something like this gets much closer to complete support:

^(?:[a-z_]+(?:\\d*[a-zA-Z_]*)*)(?:\\.[a-z_]+(?:\\d*[a-zA-Z_]*)*)*$

It verifies that "aaa" failed.

I'm changed it to

^(?:[a-zA-Z]+(?:\d*[a-zA-Z_]*)*)(?:\.[a-zA-Z]+(?:\d*[a-zA-Z_]*)*)+$

package name rules
A full Java-language-style package name for the Android app. The name may contain uppercase or lowercase letters ('A' through 'Z'), numbers, and underscores ('_'). However, individual package name parts may only start with letters.

application ID rules
And although the application ID looks like a traditional Java package name, the naming rules for the application ID are a bit more restrictive:
It must have at least two segments (one or more dots).
Each segment must start with a letter.
All characters must be alphanumeric or an underscore [a-zA-Z0-9_].

https://developer.android.com/guide/topics/manifest/manifest-element
https://developer.android.com/studio/build/application-id