The code is distributed under the BSD 2-clause license. Contributors making pull requests must agree that they are able and willing to put their contributions under that license.

Goals & non-goals of Pygments

Python support

Pygments supports all supported Python versions as per the Python Developer's Guide. Additionally, the default Python version of the latest stable version of RHEL, Ubuntu LTS, and Debian are supported, even if they're officially EOL. Supporting other end-of-life versions is a non-goal of Pygments.


Pygments does not attempt to validate the input. Accepting code that is not legal for a given language is acceptable if it simplifies the codebase and does not result in surprising behavior. For instance, in C89, accepting // based comments would be fine because de-facto all compilers supported it, and having a separate lexer for it would not be worth it.

Contribution checklist

  • Check the documentation for how to write a new lexer, a new formatter or a new filter

  • Make sure to add a test for your new functionality, and where applicable, write documentation.

  • When writing rules, try to merge simple rules. For instance, combine:

      (r"\(", token.Punctuation),
      (r"\)", token.Punctuation),
      (r"\[", token.Punctuation),
      (r"\]", token.Punctuation),
      ("{", token.Punctuation),
      ("}", token.Punctuation),


    (r"[\(\)\[\]{}]", token.Punctuation)
  • Be careful with .*. This matches greedily as much as it can. For instance, a rule like @.*@ will match the whole string @first@ second @third@, instead of matching @first@ and @second@. You can use @.*?@ in this case to stop early. The ? tries to match as few times as possible.

  • Beware of so-called "catastrophic backtracking". As a first example, consider the regular expression (A+)*C. This is equivalent to A*B regarding what it matches, but non-matches will take very long. This is because of the way the regular expression engine works. Suppose you feed it 50 'A's, and a 'C' at the end. It first matches the 'A's greedily in A+, but finds that it cannot match the end since 'B' is not the same as 'C'. Then it backtracks, removing one 'A' from the first A+ and trying to match the rest as another (A+)*. This fails again, so it backtracks further left in the input string, etc. In effect, it tries all combinations


    Thus, the matching has exponential complexity. In a lexer, the effect is that Pygments will seemingly hang when parsing invalid input.

    >>> import re
    >>> re.match('(A+)*B', 'A'*50 + 'C') # hangs

    As a more subtle and real-life example, here is a badly written regular expression to match strings:


    If the ending quote is missing, the regular expression engine will find that it cannot match at the end, and try to backtrack with less matches in the *?. When it finds a backslash, as it has already tried the possibility ., it tries . (recognizing it as a simple character without meaning), which leads to the same exponential backtracking problem if there are lots of backslashes in the (invalid) input string. A good way to write this would be r'"([^\\]|\\.)*?"', where the inner group can only match in one way. Better yet is to use a dedicated state, which not only sidesteps the issue without headaches, but allows you to highlight string escapes.

    'root': [
        (r'"', String, 'string'),
    'string': [
        (r'\\.', String.Escape),
        (r'"', String, '#pop'),
        (r'[^\\"]+', String),
  • When writing rules for patterns such as comments or strings, match as many characters as possible in each token. This is an example of what not to do:

    'comment': [
        (r'\*/', Comment.Multiline, '#pop'),
        (r'.', Comment.Multiline),

    This generates one token per character in the comment, which slows down the lexing process, and also makes the raw token output (and in particular the test output) hard to read. Do this instead:

    'comment': [
        (r'\*/', Comment.Multiline, '#pop'),
        (r'[^*]+', Comment.Multiline),
        (r'\*', Comment.Multiline),
  • Don't add imports of your lexer anywhere in the codebase. (In case you're curious about -- this file exists for backwards compatibility reasons.)

  • Use the standard importing convention: from token import Punctuation

  • For test cases that assert on the tokens produced by a lexer, use tools:

    • You can use the testcase formatter to produce a piece of code that can be pasted into a unittest file: python -m pygments -l lua -f testcase <<< "local a = 5"

    • Most snippets should instead be put as a sample file under tests/snippets/<lexer_alias>/*.txt. These files are automatically picked up as individual tests, asserting that the input produces the expected tokens.

      To add a new test, create a file with just your code snippet under a subdirectory based on your lexer's main alias. Then run pytest --update-goldens <filename.txt> to auto-populate the currently expected tokens. Check that they look good and check in the file.

      Also run the same command whenever you need to update the test if the actual produced tokens change (assuming the change is expected).

    • Large test files should go in tests/examplefiles. This works similar to snippets, but the token output is stored in a separate file. Output can also be regenerated with --update-goldens.