Skip to main content

Prepared Patterns - Introduction

Documentation for version: 0.39.0

When arbitrary value is needed in the pattern, it might be tempting to use simple string concatenation. However, integration of dynamic values such as $_GET['domain'] can be troublesome or even dangerous.

Pattern::of('^https://' . $_GET['domain'] . '\.(com|net)'); // risky and dangerous

But you, dear reader, know that's a terrible, terrible idea. $_GET['domain'] may contain unexpected/malicious regular expression special characters, which at best will match incorrect characters, malform the pattern completely, or in the worse case - open the door to ReDos attack.

Potential risks include:

  • Complex look-aheads and look-arounds ((?!<)
  • Recursive patterns ((?R))
  • Structures prone to catastrophic backtracking

Introduction#

Prepared patterns is T-Regx solution for using arbitrary, dynamic values that need to be integrated into the regular expression. Pattern instantiated with prepared patterns are safely constructed. Such construction alleviates the dangers of ReDos attacks and improper regular expression syntax.

The idea is very similar to prepared statements from SQL world, in which many SQL statements suffer from a very similar issue, yet in that case the trouble is either improper SQL syntax or even SQL-Injection vulnerability. Despite being utterly different technologies, both ReDos and SQL-Injection vulnerabilities come from the same error - improper integration of language (Regexp and SQL) and arbitrary parameters passed into it.

Regular expression template#

In order to safely integrate dynamic or unsafe values into the regular expression, special character @ must be used in the expression as a placeholder. The @ placeholder behaves like a standard regular expression figure, and matches whatever characters or figures are bound to it when prepared pattern is instantiated.

The placeholder @ matches a literal value (string), an alteration of values or more complex structures, such as other patterns or masks, depending on what figure exactly was bound to the particular placeholder. First, let's take a look at literal values.

Looking at the previous example, the initial regular expression is improperly constructed by string concatenation.

$input = $_GET['name'];
Pattern::of("(My|Our) dog's name is " . $input . '!'); // improper and risky

Naive string concatenation is risky for various reasons. Let us use Pattern::inject(), instead.

Instead of concatenating with $input, let's add placeholder @ into the regular expression, and bind $input figure to it. The placeholder @ used in the regular expression will match whatever characters or figures are bound to it. With Pattern::inject(), the bound values (called figures) are supposed to be passed as an array, as the second argument of Pattern::inject().

In T-Regx terminology, a figure is whatever is being matched by placeholder @. Scroll down, to read more about Terminology.

$input = $_GET['name'];
$pattern = Pattern::inject("(My|Our) dog's name is @!", [$input]);

Using regular expressions with @ placeholder and binding figures to it is the recommended approach, instead of naive string concatenation.

Entry point Pattern::inject() returns an instance of Pattern, exactly like Pattern::of(), so the interface of the Pattern is exactly the same.

Following the example above: Pattern::inject("(My|Our) dog's name is @!", [$input]), we find that the Pattern will match: one of "My" or "Our", then string " dog's name is " literally, then whatever was passed in $_GET['name'], and then "!" literally. Any regular expression characters, such as ., ?, (, ) that are potentially present in $_GET['name'] will be matched literally, not as a regular expression. This is the main reason why Prepared Patterns alleviate risks of malformed patterns and ReDos attacks.

Should two values be required in the pattern - a template with two @ placeholders must be used, and two figures must be passed as the second argument.

$start = $_GET['start'];
$end = $_GET['end'];
$pattern = Pattern::inject("^@:\w+?:@$", [$start, $end]);

The number of @ placeholders in the template and the number of figures bound must be equal. When too many or too little figures is bound to the template, PlaceholderFigureException is thrown and instantiation of Pattern fails.

Close examination of the constructed Pattern can be done by casting the resulting Pattern to string. Casting any instance of Pattern, regardless of whether it was created with Pattern::of(), Pattern::inject() or any other entry point, results in a delimited regular expression.

/* Pass a potentially unsafe string */
$input = "w.w?";
/* Safely instantiate the pattern */
$pattern = Pattern::inject("^'@!+$", [$input]);
/* Examine the constructed regular expression */
$regularExpression = (string) $pattern;
/* Render the regular expression, for debugging purposes */
echo $regularExpression;
/^'w\.w\?!+$/

When examining the constructed regular expression from Pattern, you will notice that special characters . and ? used in "w.w?" are represented as escaped characters \. and \? in the regular expression.

More details on placeholder @ syntax can be found in the next chapter: Template Syntax. Also, scroll to FAQ for the most common questions.

Conditional placeholders#

Placeholder @ in template behaves like all other regular expression figures, so expression @? is an optional placeholder, which matches an optional bound figure. Of course, other quantifiers such as * and + are also allowed, for example @* and @+.

$directory = "home_folder";
$filename = "image.jpg";
$directoryPattern = Pattern::inject("^/@/@?$", [$directory, $filename]);

Notice, that the second placeholder @? matches optionally, due to it being followed by the wildcard ?, by regular expression. Because of that, $directoryPattern will match either "/home_folder/" or "/home_folder/image.jpg". For curious minds, it will not match "/home_folder/image.jp".

By regular expression, @? matches the figures greedily. Analyzing the template, we see: assertion ^ matches the start of the subject, / matches character "/" literally, the first placeholder @ matches figure "home_folder", then / matches "/", then the second placeholder @ matches "image.jpg" literally, greedy quantifier ? means the previous token (which is @) matches 0 or 1 time, as much as possible, which means @? matches either "image.jpg" or nothing at all; and finally assertion $ matches the end of subject.

To conditionally match a placeholder, but not greedily, the placeholder should be followed by ??, by regular expression.

It's worth to mention, that regardless of the quantifier used for the placeholder, the figure must still be bound for the placeholder. Omitting the figure in the Pattern::inject(), while a valid @ placeholder is used in the template, even followed by ?, is disallowed.

Pattern::inject("^/@/@?$", [$directory]); // throws PlaceholderFigureException

In other words, template ^/@/@?$ consists of two placeholders, and thus two bound figures are required.

More details on usage of @ placeholder in the template be found in the next chapter: Template Syntax. Also, scroll to FAQ for the most common questions.

Pre-compiling templates#

A quite popular use-case of prepared patterns appears when a single template should be used to instantiate multiple instances of Pattern with different bound figures. Pattern::template() accepts a regular expression with placeholders, but without any bound figures. Afterwards, at a later stage, a new pattern can be instantiated from the template, with bound figures. Pattern::template() accepts a single placeholder in the template, as opposed to Pattern::builder(), which accepts varying number of placeholders.

/* Create your template */
$template = Pattern::template('^(my|your):@$');
/* Instantiate the pattern */
$pattern1 = $template->literal($_GET['value1']);
$pattern2 = $template->literal($_GET['value2']);

Modifiers can be passed as the second argument in Pattern::template(), for example Pattern::template('^(my|your):@$', 'i').

Using Pattern::template() has the added advantage of parsing and compiling the pattern early, and deferring binding of the figures for the later time. A common use-case is instantiating Pattern::template() in a constructor of your domain class, and only binding the figure in the instance method. Perhaps the method argument can be bound as a figure.

class Checker {
private Template $template;
public function __construct() {
$this->template = Pattern::template('^word:@$');
}
public function test(string $word, string $subject): bool {
$pattern = $this->template->literal($word);
return $pattern->test($subject);
}
}

If multiple patterns are only different in the values they bind, it's recommended to create a template earlier, and insert the exact values in the later stage. Instantiating a new pattern each time can be wasteful for big regular expressions:

foreach ($values as $value) {
$pattern = Pattern::inject('^@$', [$value]); // expensive
}

compared to creating the template first:

$template = Pattern::template('^@$'); // construct once
foreach ($values as $value) {
$pattern = $template->literal($value); // optimal
}

This is not to say Pattern::inject() is not to be used. If a certain template is only going to be used once, then pre-compiling it with Pattern::template() doesn't yield any performance gain and Pattern::inject() is equally viable. On the other hand, using Pattern::template() can still be beneficial to compile the template early, rather than later, for example in the domain class constructor.

Pattern::template() is a shorthand interface for Pattern::builder(). Pattern::builder() exposes an interface which binds multiple figures. Final call to bind() instantiates the constructed Pattern. Pattern::template() is a shorthand, which only accepts a single bound figure, yet it is completely sufficient, as Pattern::template() covers 95% of prepared pattern use-cases.

Pre-compiling templates with multiple figures#

Binding a single figure into the template is the majority of use cases. Should that not be the case, Pattern::builder() can be used to bind multiple values and also allows to construct the template early. The word "builder" in Pattern::builder() refers to TemplateBuilder.

/* Create your template with builder */
$template = Pattern::builder('^(my|your):@:@$');
/* Instantiate the pattern */
$pattern = $template->literal($_GET['first'])->literal($_GET['second'])->build();

The number of placeholders @ in the template and the number of chained literal() calls must be equal. When too many or too little figures is bound into the template, PlaceholderFigureException is thrown and instantiation of Pattern fails.

Calling Pattern::builder() and only using a single ->literal() is in fact identical to calling Pattern::template()->literal().

Let's take a look at the example above, but with TemplateBuilder used, instead of previous Template, so that the template can consist of two @ placeholders, with one figure being bound in the constructor, and the second in the instance method.

class Checker {
private TemplateBuilder $template;
public function __construct(string $first) {
$this->template = Pattern::builder('^@:@$')->literal($first);
}
public function test(string $second, string $subject): bool {
$pattern = $this->template->literal($word)->build();
return $pattern->test($subject);
}
}

Calls to ->literal() can be safely refactored, since Pattern::builder() is immutable.

PCRE-styled patterns#

Should there be a need for your own delimiters, PcrePattern can be used. PcrePattern offers similar entry points (PcrePattern::of(), PcrePattern::inject(), PcrePattern::list()), with the difference that the regular expressions must already be delimited and have their modifiers appended. PcrePatterns still accepts placeholders @ in their templates, for example with PcrePattern::template().

Example of PcrePattern::inject(), with pcre-styled template and figure containing the delimiter %:

$linkPattern = PcrePattern::inject("%https?://@/index\.php%m", ['one%two']);

Pattern $linkPattern can be analyzed: % is a delimiter stating the start of PCRE regular expression, http matches "http", s? matches either "s" or nothing at all, :// matches "://", placeholder @ matches "one%two" literally, index matches "index", escaped \. matches ".", php matches "php", final % states the end of regular expression, m is a modifier.

Corresponding methods PcrePattern::template() and PcrePattern::builder() work accordingly.

PcrePattern entry points return instance of Pattern, so the resulting pattern can be used as any other T-Regx Pattern.

Frequently asked questions#

  • How do I pass modifiers (flags) into Pattern::inject() and other entry points?

    • Modifiers can be passed as the third argument in Pattern::inject(). For example, passing i (case-insensitive) modifier:

      Pattern::inject('^@$', [$input], 'i');

      Remaining entry points accept modifiers as the second argument:

      Pattern::template('^@$', 'i');
      Pattern::builder('^@$', 'i');
  • Is passing an empty string as figure allowed?

    • Yes - absolutely, an empty string is a perfectly valid figure. For example, template <@> with bound figure of an empty string: Pattern::inject('<@>', [""]) - matches string "<>" just fine.

      Furthermore, placeholders @? with quantifiers also work with empty strings. For example:

      $wordPattern = Pattern::inject("^word:@?$", [""]);

      In this example, $wordPattern matches subject "word:", but not "word". For the pattern to match "word", an alternative template is needed, namely: ^word(:@)?$.

      In other words, if the bound figure matches 0 characters, by regular expression, so does the corresponding @ placeholder.

  • Is passing an empty array of figures to Pattern::inject() allowed?

    • If the template doesn't contain any placeholders @, then actually an empty array is the only allowed input, although it's identical to simply calling Pattern::of().

      By analogy, the same applies to Pattern::builder(). When template doesn't contain any @ placeholder, then no chain of ->literal() is allowed before ->bind(). Needless to say, such construct is identical to Pattern::of() and is only present in the API for completeness.

  • Is Pattern::of($p) identical to Pattern::inject($p, [])?

    • Yes, provided that $p doesn't contain unescaped @ special character (which would be a placeholder in Pattern::inject()).
  • How can character "@" be used literally in the pattern?

    • Escaped character \@ is not treated as a placeholder, for example:

      $username = "mark";
      $mailPattern = Pattern::inject("@\@gmail.com", [$username]);

      In the example above, Pattern in $mailPattern matches subject "mark@gmail.com".

      More details on when exactly @ is a placeholder can be found in the next chapter: Template Syntax.

  • What happens when I pass string "@" as a figure. Will it recurse placeholders?

    • No, character "@" passed as a figure will be treated as another literal, for example:
      Pattern::inject('^@:@$', ['word', '@']);
      Instance of the Pattern above will match subject "word:@", though it won't construct optimal regular expression. Escaping the @ character is the recommended approach:
      Pattern::inject('^@:@$', ['word', '@']); // not optimal
      Pattern::inject('^@:\@$', ['word']); // optimal
      Escaping @ character is not only optimal performance-wise, but is also more readable.
  • Can figures influence the construction of the Pattern, for example by taking part in the selection of the delimiter?

    • No, the template is assigned a delimiter regardless of the figures bound to it. For example:

      $value = "/"; // slash, a standard delimiter
      /* Render the regular expression, for debugging purposes */
      echo (string) Pattern::inject('^@$', [$value]);

      constructs a pattern with standard / delimiter, with figure "/" being properly escaped:

      /^\/$/

What about special cases#

T-Regx prepared patterns understand that sometimes @ figure shouldn't be a placeholder, even when using Pattern::inject() or Pattern::template().

Structures, in which character @ is not a placeholder (matches "@" literally):

  • Escaped character: \w+:\@
  • Character class: \w+:[0-9@]
  • Perl quote: \w+:\Q@\E
  • Control character: \w+:\c@
  • Comment group: \w(?#comment@)
  • Comment (when x flag is used): \w+:#@\n

Other cases, when @ placeholder can be used are:

  • Capturing groups: (word:@) (figure bound to @ is captured)
  • Non-capturing groups: (?:word:@) (figure bound to @ is not captured)
  • Atomic capturing groups: (?>word:@)
  • Look-aheads: (?="@")
  • Look-behinds: (?<="@")

More details on when exactly @ is a placeholder can be found in the next chapter: Template Syntax.

Terminology#

There are 4 new terms introduced with prepared patterns in this chapter: "Prepared patterns", "template", "placeholder" and "figure".

  • "Prepared patterns" is a common name for the interface for safe construction of instances of Pattern with arbitrary values (like dynamic strings). In this documentation, entry points Pattern::inject() and Pattern::template() are referred to as "prepared patterns". In other words, "prepared patterns" are patterns with @ placeholder in them.

    Pattern::of() is not referred to as "prepared pattern", because while character "@" can very well be used in the regular expression with of() (for example Pattern::of('\w+@\w+.\w+')), it is not a placeholder. Character "@" used in Pattern::of() matches character "@" literally, where-as placeholders @ in Pattern::inject() and Pattern:template() will match the values that will have been bound to them.

    More details on when exactly @ is a placeholder can be found in the next chapter: Template Syntax.

  • "template" is a name for a regular expression that contains @ placeholder in it, and can later be bound to certain figures or character which will match in place of @ placeholder. Entry points Pattern::inject() and Pattern::template() accept template as their first argument.

    More details on when exactly @ is a placeholder can be found in the next chapter: Template Syntax.

  • "placeholder" is a name for a figure @ appearing in a template. Not every occurrence of @ in a regular expression is a placeholder. For example, escaped \@ is not a placeholder, and so will match character "@" literally.

    More details on when exactly @ is a placeholder can be found in the next chapter: Template Syntax.

  • "figure" is a name for anything that can be bound to a placeholder, most common of which is a literal string, but can also be an alteration, another pattern or a mask.

Last updated on