Modified Regular Expressions

The copyright scanner has to meet several competing requirements: 1) it must be fast to keep server load and latency reasonable, 2) it must find copyrights and licenses appearing in and interrupted by any number of comment formats, and 3) it must be configurable by mere mortals.

Configured patterns are basically regular expressions with modifications to assist the 3 requirements above.

Unlimited Wildcards

If you run the command-line tool against the files in your repository, it will already find a lot of the copyright and license declarations that interest. Often it will pick up a couple extra junk words before or after the text you want. In these cases, the natural inclination to use wildcards: e.g. something like .*My License.* in a first-party pattern. However, when scanning a large file, those wildcards can pick up a lot more junk words and slow the scan to a crawl.

To allow the easy and obvious choice, the plugin allows you to use the .* and .+ patterns, and it modifies your pattern before scanning. When they appear at the start or the end of the pattern, the plugin removes them before scanning the file and puts them back when classifying the matches.

The scanner will still find all of the matches containing the pattern that it found before plus it might find a few exact matches without increasing the work to scan the file. The matches are limited in size. When it puts the wildcards back to analyze the matches, the wildcards in your first-party pattern cause it to match despite the junk words so it correctly identifies the first-party license without costing much cpu time.

When they appear in the middle of your pattern, the plugin replaces them with a pattern that matches a limited number of words separated by a limited number of whitespace characters. The limits are large enough generally to match anything of interest, but small enough to keep the scan down to a reasonable time.

If the limits cause the plugin to miss too many desired hits, it's always possible to write a more complex pattern with different limits.

Comment Characters and Whitespace

Whitespace does not affect the meaning of a license. The same text may be formatted all manner of different ways using spaces, tabs, newlines, etc. to align margins, center text, word-wrap at different columns etc. without changing the meaning. Many times the same text will be incorporated into comments of different languages. Even in the same language, sometimes the author will use multi-line comments /.../ and sometimes the author will use single-line comments //. Some languages use a completely different comment character #.

You could insert a complex expression anywhere you expect whitespace. e.g. [\\s/*#]+ But the configuration will already be unreadable, and if a file has a lot of whitespace, that pattern could match large blocks while slowing the scan.

To keep configurations readable, the plugin substitutes any embedded spaces with a regular expression to match a limited number of whitespace or comment characters where the limit is long enough to match almost all of the potential hits without slowing the scan too much.

Mostly you don't need to worry about the details. Just use a space between words even if they might appear on different lines etc.

However, if you use a space inside a character class [ a-z0-9], the plugin will reject the pattern. You need to break that out into a more complicated pattern: (?: |[a-z0-9])

Capture Groups

The plugin uses capture groups to keep track of licenses versus owners. To group parts of your patterns, use a non-capturing group (?:pattern) instead of (pattern). The plugin will reject your configuration if you try to use a pattern that looks like it contains a capture group.

This might also force you to use a different order in character classes: e.g. [)a-d(] instead of [(a-d)]