Update regex note

The regex note applies generally to Zuul configuration syntax, so
move it to the general page.  To make it more relevant to each of
the items which it specifically affects, hyperlink regex references
to the note.

Change-Id: Ia971f5d52496fd4ccdfa18db4c9e3d39c1351bdc

doc/source/reference/config.rst

@@ -59,6 +59,17 @@ As soon as a change containing a Zuul configuration change merges to
 any Zuul-managed repository, the new configuration takes effect
 immediately.
 
+.. _regex:
+
+Regular Expressions
+-------------------
+
+Many options accept literal strings or regular expressions.  In these
+cases, the regular expression matching starts at the beginning of the
+string as if there were an implicit ``^`` at the start of the regular
+expression. To match at an arbitrary position, prepend ``.*`` to the
+regular expression.
+
 .. _configuration-items:
 
 Configuration Items

doc/source/reference/job_def.rst

@@ -78,13 +78,6 @@ Here is an example of two job definitions:
            - name: test-node
              label: fedora
 
-.. warning::
-
-   Regular expression matching in Zuul *starts at the beginning of
-   the string*, i.e., as if there was an implicit ``^`` at the beginning
-   of the regular expression. To match at an arbitrary position, prepend
-   ``.*`` to the regular expression.
-
 .. attr:: job
 
    The following attributes are available on a job; all are optional
@@ -796,9 +789,10 @@ Here is an example of two job definitions:
 
    .. attr:: branches
 
-      A regular expression (or list of regular expressions) which
-      describe on what branches a job should run (or in the case of
-      variants, to alter the behavior of a job for a certain branch).
+      A :ref:`regular expression <regex>` (or list of regular
+      expressions) which describe on what branches a job should run
+      (or in the case of variants, to alter the behavior of a job for
+      a certain branch).
 
       This attribute is not inherited in the usual manner.  Instead,
       it is used to determine whether each variant on which it appears
@@ -864,7 +858,7 @@ Here is an example of two job definitions:
       specified files are modified.  Unlike **branches**, this value
       is subject to inheritance and overriding, so only the final
       value is used to determine if the job should run. This is a
-      regular expression or list of regular expressions.
+      :ref:`regular expression <regex>` or list of regular expressions.
 
    .. attr:: irrelevant-files
 
@@ -872,8 +866,8 @@ Here is an example of two job definitions:
       the job should run unless *all* of the files changed match this
       list.  In other words, if the regular expression ``docs/.*`` is
       supplied, then this job will not run if the only files changed
-      are in the docs directory.  A regular expression or list of
-      regular expressions.
+      are in the docs directory.  A :ref:`regular expression <regex>`
+      or list of regular expressions.
 
    .. attr:: match-on-config-updates
       :default: true

doc/source/reference/pragma_def.rst

@@ -38,7 +38,7 @@ pragma directives may not be set and then unset within the same file.
 
    .. attr:: implied-branches
 
-      This is a list of regular expressions, just as
+      This is a list of :ref:`regular expressions <regex>`, just as
       :attr:`job.branches`, which may be used to supply the value of
       the implied branch matcher for all jobs and project-templates in
       a file.

doc/source/reference/tenants.rst

@@ -305,18 +305,18 @@ configuration. Some examples of tenant definitions are:
    .. attr:: allowed-labels
       :default: []
 
-      The list of labels (as strings or regular expressions) a tenant
-      can use in a job's nodeset. When set, this setting can be used
-      to restrict what labels a tenant can use.  Without this setting,
-      the tenant can use any labels.
+      The list of labels (as strings or :ref:`regular expressions <regex>`)
+      a tenant can use in a job's nodeset. When set, this setting can
+      be used to restrict what labels a tenant can use.  Without this
+      setting, the tenant can use any labels.
 
    .. attr:: disallowed-labels
       :default: []
 
-      The list of labels (as strings or regular expressions) a tenant
-      is forbidden to use in a job's nodeset. When set, this setting
-      can be used to restrict what labels a tenant can use.  Without
-      this setting, the tenant can use any labels permitted by
+      The list of labels (as strings or :ref:`regular expressions <regex>`)
+      a tenant is forbidden to use in a job's nodeset. When set, this
+      setting can be used to restrict what labels a tenant can use.
+      Without this setting, the tenant can use any labels permitted by
       :attr:`tenant.allowed-labels`.  This check is applied after the
       check for `allowed-labels` and may therefore be used to further
       restrict the set of permitted labels.