Manual:$wgLegalTitleChars

Details
This is a regex character class (i.e. a list of characters in a format suitable for a regular expression) that you want MediaWiki to allow in meta>m:Special:MyLanguage/Help:Page_name|page titles despite being in the list of illegal characters.

The list of illegal characters is as follows:  , non-printable characters 0 through 31, and 'delete' character 127.

Problem characters
The following punctuation symbols may cause problems if enabled:


 *   - These are needed for link and template syntax, never enable them.
 *   - These cause problems with HTML escaping, don't use.
 *   - Minor problems with path to query rewrite rules, see below. Included in the default allow list.
 *   - Doesn't work with path to query rewrite rules, corrupted by apache. Included in the default allow list since MediaWiki 1.8.0.  In some rare cases you may wish to remove + for compatibility with old links.
 *   - Doesn't work with path to PATH_INFO rewrites. Included in the default allow list.

The last three of these punctuation problems can be avoided by using an alias, instead of a rewrite rule of either variety.

The problem with % is that when using a path to query rewrite rule, URLs are double-unescaped: once by Apache's path conversion code, and again by PHP. So %253F, for example, becomes "?". Our code does not double-escape to compensate for this, indeed double escaping would break if the double-escaped title was passed in the query string rather than the path. This is a minor security issue because articles can be created such that they are hard to view or edit.

Theoretically 0x80-0x9F of ISO 8859-1 should be disallowed, but this breaks interlanguage links and so they are included in the allowed list by default.