E621's built-in tag lookup is adequate for everyday usage, but its search options are woefully lacking when it comes to finding tags that match specific patterns, rather than simple matching.

This is where this utility comes in. The pattern search input lets you specify a regular expression that the tags should match. It follows the fairly standard regex rules.

[abc] A single character: a, b or c
[^abc] Any single character but a, b, or c
[a-z] Any single character in the range a-z
[a-zA-Z] Any single character in the range a-z or A-Z
^ Start of line
$ End of line
\A Start of string
\z End of string
. Any single character
\s Any whitespace character
\S Any non-whitespace character
\d Any digit
\D Any non-digit
\w Any word character (letter, number, underscore)
\W Any non-word character
\b Any word boundary character
(...) Capture everything enclosed
(a|b) a or b
a? Zero or one of a
a* Zero or more of a
a+ One or more of a
a{3} Exactly 3 of a
a{3,} 3 or more of a
a{3,6} Between 3 and 6 of a

The custom searches are used to filter data that cannot be matched with a simple pattern.

Unicode Characters: tags that contain unicode characters in them. Such tags are no longer valid on e621, and should be replaced.

Invalid Characters: tags that contain any invalid character. This includes unicode characters, a leading tilde / underscore / minos / colon, peso signs, percent signs, and some others. A standalone implementation of this filter is available here.

Negative Count: tags with a negative post count. This usually happens if the tag gets removed from deleted posts, and requires a janitor's intervention to fix.

Note that this utility uses a cached version of e621's tags. This means that the changes you make will not be immediately reflected in this list.

A new copy of the tag list is fetched daily.