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Transforms text by taking capture groups matched by a [RegExp] and outputting them as plain text and/or evaluating them as expressions, as defined by an output template.

regex_transformer #

Transforms text by taking capture groups matched by a [RegExp] and outputting them as plain text and/or evaluating them as expressions, as defined by an output template.

Usage #

import 'package:regex_transformer/regex_transformer.dart';

[RegexTransformer] has two methods: transform and transformAll.

transform matches the [RegExp] with the first match found in the input text and returns the output template with the matched capture groups.

transformAll takes every [RegExp] match found in the input text and replaces each match with that match's parsed output.

Capture Groups #

A [RegExp]'s capture groups can be output by annotating them in the output template with a $ followed by the name or index of the capture group.

Note: Because the $ character used to inject variables into strings in dart, output templates should be provided as a raw string (r'...'), otherwise every $ annotation will have to be escaped (\$).

Indexed capture groups #

/// This transformer defines 3 indexed capture groups and
/// returns them as defined by the output template.
final transformer = RegexTransformer(
  regex: RegExp(r'(.*) .* (.*) .* (.*)'),
  output: r'$1 + $2 = $3',
);

// one + two = fish
print(transformer.transform('one plus two equals fish'));

Note: $0 can be used to output the entire match.

/// This transformer wraps the entire match in parentheses.
final transformer = RegexTransformer(
  regex: RegExp(r'[a-z]+'),
  output: r'($0)',
);

// (one) (plus) (two) (equals) (fish)
print(transformer.transformAll('one plus two equals fish'));

Named capture groups #

/// This transformer defines 3 indexed capture groups and
/// returns them as defined by the output template.
final transformer = RegexTransformer(
  regex: RegExp(r'(?<one>.*) .* (?<two>.*) .* (?<three>.*)'),
  output: r'$one + $two = $three',
);

// one + two = fish
print(transformer.transform('one plus two equals fish'));

Note: Named capture groups are also assigned an index, so keep in mind if using a combination of named and indexed capture group annotations that the indexed annotations must factor the named capture groups into the count.

Escapes #

A backslash (\) can be used to escape a $ character to have it output as plain text, rather than to annotate a capture group, and can also be used to break a capture group's ID, allowing it to be output next to other alphanumeric characters without a space or other character.

Note: Backslashes will have to be escaped with another backslash (\\) to have one output anywhere in your output template as plain text, regardless of whether it's being used as an escape or not.

/// This transformer utilizes escapes to break the capture groups' IDs, so
/// they can be output directly next to other alphanumeric characters.
final transformer = RegexTransformer(
  regex: RegExp(r'(?<one>.*) .* (?<two>.*) .* (?<three>.*)'),
  output: r'$one\plus$two\equals\$three',
);

// oneplustwoequals$three
print(transformer.transform('one plus two equals fish'));

Expressions #

Expressions can be defined in output templates and evaluated by the transformation methods, and can handle most standard operations.

Like capture groups, expression are annotated with a $, but are wrapped with parentheses: $(...).

Note: Expressions are parsed and evaluated utilizing the expressions package.

/// This transformer evaluates an expression and outputs the result.
final transformer = RegexTransformer(
  regex: RegExp(r'(?<one>[0-9]).*(?<two>[0-9])'),
  output: r'$one + $two = $($one + $two)',
);

// 2 + 3 = 5
print(transformer.transform('2 + 3 = fish'));

Variables & Functions #

Variables and functions can be provided to the expression evaluator's context to parse and evaluate them as part of the expression.

/// Expressions defined in this transformer's output template can
/// utilize the `ceil` and `combine` functions provided to it.
final transformer = RegexTransformer(
  regex: RegExp(r'([0-9]) \+ ([0-9]) = ([a-z]*)'),
  variables: {
    'ceil': (double input) => input.ceil(),
    'combine': (String input) => input.codeUnits.reduce((a, b) => a + b),
    'oneHundred': 100,
  },
  output: r'$1 + $2 = $(ceil(combine($3) / oneHundred))',
);

// 2 + 3 = 5
print(transformer.transform('2 + 3 = fish'));
/// This transformer reverses the matched word.
final transformer = RegexTransformer(
  regex: RegExp(r'[a-z]+'),
  variables: {'reverse': (String input) => input.split('').reversed.join()},
  output: r'$(reverse($0))',
);

/// eno + owt = hsif
print(transformer.transformAll('one + two = fish'));

Math #

By setting [RegexTransformer]'s math parameter to true, every constant and function in the dart:math library, as well as [num]'s abs, round, ceil, and floor methods will be provided to the expression evaluator's context and can be utilized within expressions.

/// Expressions defined in this transformer's output template can
/// evaluate the `dart:math` library's constants and functions.
final transformer = RegexTransformer(
  regex: RegExp(r'([0-9]) \+ ([0-9]) = ([a-z]*)'),
  output: r'$3 = $(round((sin(($2 * pi) * ($2 / $1)) +'
      r'cos(($1 * pi) * ($1 / $2))) * 10))',
  math: true,
);

// fish = 5
print(transformer.transform('2 + 3 = fish'));

Strict Transformers #

Setting [RegexTransformer]'s strict parameter to true will result in exceptions being thrown should there be any errors while parsing the output template or the input text.

If any capture groups annotated in the output template aren't matched in the text input into the transformation methods an [ArgumentError] will be thrown, while a [FormatException] will be thrown if an expression can't be parsed or evaluated.

/// Any text transformed by this transformer will throw an [ArgumentError] as
/// the [RegExp]'s capture groups aren't named, but the output template calls
/// for named capture groups.
final transformer = RegexTransformer(
  regex: RegExp(r'(.*) .* (.*) .* (.*)'),
  output: r'$one + $two = $three',
  strict: true,
);

Extension Methods #

This package also extends [String] and [RegExp] with [RegexTransformer]'s transform and transformAll methods.

Note: These methods are intended to be used for one-off transformations. If transforming multiple inputs with the same [RegExp] and output template, it's computationally more efficient to create a [RegexTransformer], as these methods parse the output template every time they're called, while a [RegexTransformer] will only parse the output template once upon initialization.

String #

final myString = 'one plus two equals fish';

// one + two = fish
print(myString.transform(RegExp(r'(.*) .* (.*) .* (.*)'), r'$1 + $2 = $3'));

// (one) (plus) (two) (equals) (fish)
print(myString.transformAll(RegExp(r'[a-z]+'), r'($0)'));

RegExp #

final myRegex = RegExp(r'(.*) .* (.*) .* (.*)');

// one + two = fish
print(myRegex.transform('one plus two equals fish', r'$1 + $2 = $3'));
final myRegex = RegExp(r'[a-z]+');

// (one) (plus) (two) (equals) (fish)
print(myRegex.transformAll('one plus two equals fish', r'($0)'));
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Transforms text by taking capture groups matched by a [RegExp] and outputting them as plain text and/or evaluating them as expressions, as defined by an output template.

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