ReBuild

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.

-- Jamie Zawinski

With ReBuild you can write performant regex patterns in a readable and maintainable way by using functions. It is inspired by the Simple Regex Language (SRL - Simple Regex Language).

Why Rebuild?

• Helps you write maintainable regex patterns, that are readable, regardless of how good you are at regex

• ReBuild creates highly optimised regex patterns by analysing the input patterns

• It uses non-capturing groups as often as possible to enhance performance

• Fully string based: It's versatile and can be used with many different tools

• Lets you use your beloved autocomplete for generating regex patterns

• Helps you discover new ways of writing regex patterns, that involve more code reuse

Optimisation

ReBuild does a lot of processing ahead of time (AOT) to generate performant and simple regex patterns. Many of its functions can optimise specific cases, which means that many functions provide zero-cost abstraction.

A good example to illustrate the optimisation power is the either function, which represents a regex OR function, e.g. a|b|c.

# Matches the string abc, 123, or xyz
either("abc", "123", "xyz")
>>> '(?:abc|123|xyz)'


# Matches any letter from a to z, any digit or the string "xyz"
either("[a-z]", "[0-9]", "xyz")
>>> '(?:[a-z0-9]|xyz)'
# ReBuild detects the two character sets and combines them

# Matches any letter from a to z or any digit
either("[a-z]", "[0-9]")
>>> '[a-z0-9]'
# It detects that a OR is no longer necessary after 
# combining the character sets, and optimises it away


# Matches the character "a", "b" or "c"
either("a", "b", "c")
>>> '[abc]'
# ReBuild transforms the OR of individual characters into a single character set

How it works

For every function (like either, one_of, ...) ReBuild does the following steps:

1. Generate a non-optimised regex pattern (rebuild.builder)

2. Parse this pattern and convert it to a tree structure (rebuild.parser)

3. Analyse and optimise the tree (rebuild.analyser)

4. Convert the tree back to a string regex pattern

As you can see from the aforementioned steps, ReBuild consists of 3 modules: builder, parser and analyser

• rebuild.builder provides the regex building functions like either, one_of, ... which first create a non-optimised pattern, but then run them through an intermediate optimisation process

  print(either("a", "b", one_of("0-9"))
  # Before optimising (intermediate optimisations disabled for this example)
  >>> '(?:(?:a)|(?:b)|(?:[0-9]))'

• rebuild.parser parses string regex patterns with the amazing parsing library for Python Lark.

  First, it converts the regex pattern into a concrete syntax tree (CST)

  # CST of '(?:(?:a)|(?:b)|(?:[0-9]))'
  
  alternation
    single_char    a
    single_char    b
    char_set
      range    0-9

  Then this tree is converted into the another tree, now consisting of RegexNodes, which are defined in the rebuild.analyser module. Amongst other things, these nodes contain more information about the pattern (AST).

  Alternation
  |   Single Char: "a"
  |   Single Char: "b"
  |   Char Set
  |   |   Is Inverted: False
  |   |   Options
  |   |   |   Range
  |   |   |   |   From Char: "0"
  |   |   |   |   To Char: "9"

• rebuild.analyser defines the nodes of this tree, which each have rules for optimisations and methods for converting back to a string regex pattern.

  # After optimisation
  
  Char Set
  |   Is Inverted: False
  |   Options
  |   |   Single Char: "a"
  |   |   Single Char: "b"
  |   |   Range
  |   |   |   From Char: "0"
  |   |   |   To Char: "9"

  Converted back to a string:

  '[ab0-9]'

As you can see, ReBuild re-optimises the input string for every function, maximising readability and ease of use.

Should you be worried about the performance penalty of re-optimising for every single function?

No. ReBuild uses Lark's most optimised mode, a tree-less LALR(1) parser. Lark is the fastest parsing library available for Python. And after all, you should only generate the regex pattern once, store it in a constant and only use the constant for the rest of your program.

But if you want to only optimise once, you can do the following:

import rebuild.builder

rebuild.builder.INTERMEDIATE_OPTIMISATION = False


from rebuild.builder import *

print(either("a", "b", "c"))
# (?:(?:a)|(?:b)|(?:c))

print(optimise(either("a", "b", "c")))
# [abc]