Common regular expressions

Maybe the most important feature of Logos is its ability to accept regex patterns in your tokens' definition.

Regular expressions, or regexes for short, are sequences of characters (or bytes) that define a match pattern. When constructing lexers, this is especially useful to define tokens that should match a set of similar literals. E.g., a sequence of 3 ASCII uppercase letters and 3 digits could define a license plate, and could be matched with the following regex: "[A-Z]{3}[0-9]{3}".

For more details about regexes in Rust, refer to the regex crate.

Valid regexes that are not supported

Because Logos aims at generating high-performance code, it never allows to do backtracking. This means that anytime a byte is read from the input source, it will never be read again. This implementation choice comes at a cost: not all valid regexes are supported by Logos1.

For reference, Logos parses regexes using regex-syntax = 0.6, and transforms its high-level intermediate representation (HIR) into some medium intermediate representation (MIR). From HIR, MIR does not support the following HirKinds:

  • Non-greedy repetitions, i.e., matching as little as possible as given pattern.
  • ".*" and ".+" repetition patterns, because they will potentially consume all the input source, breaking the non-backtracking rule. For solutions, see footnote1 or read the error message.
  • Word boundaries, i.e., r"\b".
  • Anchors, because input source does not treat lines separately.

Additionally, note that capture groups will silently be ungrouped, because Logos does not support capturing groups, but the main slice (lex.slice()).


Most of time, however, it is possible to circumvent this issue by rewriting your regex another way, or by using callbacks. E.g., see #302.

Other issues

Logos' support for regexes is not yet complete, and errors can still exist. Some are found at compile time, and others will create wrong matches or panic.

If you ever feel like your patterns do not match the expected source slices, please check the GitHub issues. If no issue covers your problem, we encourage you to create a new issue, and document it as best as you can so that the issue can be reproduced locally.