Crate grex

Expand description

1. What does this tool do?

grex is a library as well as a command-line utility that is meant to simplify the often complicated and tedious task of creating regular expressions. It does so by automatically generating a single regular expression from user-provided test cases. The resulting expression is guaranteed to match the test cases which it was generated from.

This project has started as a Rust port of the JavaScript tool regexgen written by Devon Govett. Although a lot of further useful features could be added to it, its development was apparently ceased several years ago. The plan is now to add these new features to grex as Rust really shines when it comes to command-line tools. grex offers all features that regexgen provides, and more.

The philosophy of this project is to generate the most specific regular expression possible by default which exactly matches the given input only and nothing else. With the use of command-line flags (in the CLI tool) or preprocessing methods (in the library), more generalized expressions can be created.

The produced expressions are Perl-compatible regular expressions which are also compatible with the regular expression parser in Rust’s regex crate. Other regular expression parsers or respective libraries from other programming languages have not been tested so far, but they ought to be mostly compatible as well.

2. Do I still need to learn to write regexes then?

Definitely, yes! Using the standard settings, grex produces a regular expression that is guaranteed to match only the test cases given as input and nothing else. This has been verified by property tests. However, if the conversion to shorthand character classes such as \w is enabled, the resulting regex matches a much wider scope of test cases. Knowledge about the consequences of this conversion is essential for finding a correct regular expression for your business domain.

grex uses an algorithm that tries to find the shortest possible regex for the given test cases. Very often though, the resulting expression is still longer or more complex than it needs to be. In such cases, a more compact or elegant regex can be created only by hand. Also, every regular expression engine has different built-in optimizations. grex does not know anything about those and therefore cannot optimize its regexes for a specific engine.

So, please learn how to write regular expressions! The currently best use case for grex is to find an initial correct regex which should be inspected by hand if further optimizations are possible.

3. Current features

literals
character classes
detection of common prefixes and suffixes
detection of repeated substrings and conversion to {min,max} quantifier notation
alternation using | operator
optionality using ? quantifier
escaping of non-ascii characters, with optional conversion of astral code points to surrogate pairs
case-sensitive or case-insensitive matching
capturing or non-capturing groups
optional anchors ^ and $
fully compliant to newest Unicode Standard 14.0
fully compatible with regex crate 1.6.0+
correctly handles graphemes consisting of multiple Unicode symbols
reads input strings from the command-line or from a file
produces more readable expressions indented on multiple using optional verbose mode
optional syntax highlighting for nicer output in supported terminals

4. How to use?

The code snippets below show how to use the public api.

For more detailed examples, please take a look at the project’s readme file on GitHub.

4.1 Default settings

Test cases are passed either from a collection via RegExpBuilder::from() or from a file via RegExpBuilder::from_file().

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["a", "aa", "aaa"]).build();
assert_eq!(regexp, "^a(?:aa?)?$");

4.2 Convert to character classes

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["a", "aa", "123"])
    .with_conversion_of_digits()
    .with_conversion_of_words()
    .build();
assert_eq!(regexp, "^(?:\\d\\d\\d|\\w(?:\\w)?)$");

4.3 Convert repeated substrings

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["aa", "bcbc", "defdefdef"])
    .with_conversion_of_repetitions()
    .build();
assert_eq!(regexp, "^(?:a{2}|(?:bc){2}|(?:def){3})$");

By default, grex converts each substring this way which is at least a single character long and which is subsequently repeated at least once. You can customize these two parameters if you like.

In the following example, the test case aa is not converted to a{2} because the repeated substring a has a length of 1, but the minimum substring length has been set to 2.

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["aa", "bcbc", "defdefdef"])
    .with_conversion_of_repetitions()
    .with_minimum_substring_length(2)
    .build();
assert_eq!(regexp, "^(?:aa|(?:bc){2}|(?:def){3})$");

Setting a minimum number of 2 repetitions in the next example, only the test case defdefdef will be converted because it is the only one that is repeated twice.

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["aa", "bcbc", "defdefdef"])
    .with_conversion_of_repetitions()
    .with_minimum_repetitions(2)
    .build();
assert_eq!(regexp, "^(?:bcbc|aa|(?:def){3})$");

4.4 Escape non-ascii characters

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["You smell like 💩."])
    .with_escaping_of_non_ascii_chars(false)
    .build();
assert_eq!(regexp, "^You smell like \\u{1f4a9}\\.$");

Old versions of JavaScript do not support unicode escape sequences for the astral code planes (range U+010000 to U+10FFFF). In order to support these symbols in JavaScript regular expressions, the conversion to surrogate pairs is necessary. More information on that matter can be found here.

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["You smell like 💩."])
    .with_escaping_of_non_ascii_chars(true)
    .build();
assert_eq!(regexp, "^You smell like \\u{d83d}\\u{dca9}\\.$");

4.5 Case-insensitive matching

The regular expressions that grex generates are case-sensitive by default. Case-insensitive matching can be enabled like so:

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["big", "BIGGER"])
    .with_case_insensitive_matching()
    .build();
assert_eq!(regexp, "(?i)^big(?:ger)?$");

4.6 Capturing Groups

Non-capturing groups are used by default. Extending the previous example, you can switch to capturing groups instead.

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["big", "BIGGER"])
    .with_case_insensitive_matching()
    .with_capturing_groups()
    .build();
assert_eq!(regexp, "(?i)^big(ger)?$");

4.7 Verbose mode

If you find the generated regular expression hard to read, you can enable verbose mode. The expression is then put on multiple lines and indented to make it more pleasant to the eyes.

use grex::RegExpBuilder;
use indoc::indoc;

let regexp = RegExpBuilder::from(&["a", "b", "bcd"])
    .with_verbose_mode()
    .build();

assert_eq!(regexp, indoc!(
    r#"
    (?x)
    ^
      (?:
        b
        (?:
          cd
        )?
        |
        a
      )
    $"#
));

4.8 Disable anchors

By default, the anchors ^ and $ are put around every generated regular expression in order to ensure that it matches only the test cases given as input. Often enough, however, it is desired to use the generated pattern as part of a larger one. For this purpose, the anchors can be disabled, either separately or both of them.

use grex::RegExpBuilder;

let regexp = RegExpBuilder::from(&["a", "aa", "aaa"])
    .without_anchors()
    .build();
assert_eq!(regexp, "a(?:aa?)?");

5. How does it work?

A deterministic finite automaton (DFA) is created from the input strings.
The number of states and transitions between states in the DFA is reduced by applying Hopcroft’s DFA minimization algorithm.
The minimized DFA is expressed as a system of linear equations which are solved with Brzozowski’s algebraic method, resulting in the final regular expression.

Structs

RegExpBuilder

This struct builds regular expressions from user-provided test cases.