Allow optional regex check on first safe token value

Problem/Motivation

When using the pathauto module in combination with System tags the generation of paths doesn't work as expected. E.g. when you create a jobs overview page (node) with the following menu structure:

* Careers
** Jobs

When you create a system_tag "jobs" and add an entity_reference field (to system_tag entities) on the jobs page and reference to the "jobs" system tag, the system_tags module will create the token [system-tags][node--jobs]. This token will return the path of the jobs overview page e.g. careers/jobs.

When you use this token to generate paths for a job content type and add use the pattern "[system-tags][node--jobs]/[node:title]". The expected value is "/careers/jobs/vacancy-title". But the value we currently get is /careersjobs/vacancy-title. A possible workaround is to add every last part of the system tag tokens to the safe_tokens. An easier solution for us is to add "system-tags" to the safe_tokens.

Proposed resolution

Allow the safe_token to check the first part of of the token.