Possessive Quantifiers (Page 18)

When working with repetition operators (also known as quantifiers) in regular expressions, it’s essential to understand the difference between greedy, lazy, and possessive quantifiers. Greedy and lazy quantifiers affect the order in which the regex engine tries to match permutations of the pattern. However, both types still allow the regex engine to backtrack through the pattern to find a match. Possessive quantifiers take a different approach—they do not allow backtracking once a match is made, which can impact performance and alter match results.

How Possessive Quantifiers Work

Possessive quantifiers are a feature of some modern regex engines, including JGsoft, Java, and PCRE. These quantifiers behave like greedy quantifiers by attempting to match as many characters as possible. However, once a match is made, possessive quantifiers lock in the match and refuse to give up characters during backtracking.

You can make a quantifier possessive by adding a + after it:

• * (greedy) matches zero or more times.
• *? (lazy) matches as few times as possible.
• *+ (possessive) matches zero or more times but refuses to backtrack.

Other possessive quantifiers include ++, ?+, and {n,m}+.

Example of Possessive Quantifiers in Action

Consider the regex pattern "[^"]*+" applied to the string "abc":

1. The first " matches the opening quote.
2. The [^\"]*+ matches the characters abc within the quotes.
3. The final " matches the closing quote.

In this case, the possessive quantifier behaves similarly to a greedy quantifier. However, if the string lacks a closing quote, the regex will fail faster with a possessive quantifier because there are no backtracking steps to try.

For instance, when applied to the string "abc, the possessive quantifier prevents the regex engine from backtracking to try alternate matches, immediately resulting in a failure when it encounters the missing closing quote. In contrast, a greedy quantifier would continue backtracking unnecessarily, trying to find a match.

When Possessive Quantifiers Matter

Possessive quantifiers are particularly useful for optimizing regex performance by preventing excessive backtracking. This is especially valuable in cases where:

• You expect a match to fail.
• The pattern includes nested quantifiers.

By using possessive quantifiers, you can reduce or eliminate catastrophic backtracking, which can slow down your regex significantly.

How Possessive Quantifiers Can Change Match Results

Possessive quantifiers can alter the outcome of a match. For example:

• The pattern ".*" applied to the string "abc"x will match "abc".
• The pattern ".*+" applied to the same string will fail to match because the possessive quantifier locks in the entire string, including the extra character x, preventing the second quote from matching.

This demonstrates that possessive quantifiers should be used carefully. The part of the pattern that follows the possessive quantifier must not be able to match any characters already consumed by the quantifier.

Using Atomic Grouping Instead of Possessive Quantifiers

Atomic groups offer a similar function to possessive quantifiers. They prevent backtracking within the group, making them a useful alternative for regex flavors that don’t support possessive quantifiers.

To create an atomic group, use the syntax (?>X*) instead of X*+. For example:

• (?:a|b)*+ is equivalent to (?>(?:a|b)*).

The key difference is that the quantified token and the quantifier must be inside the atomic group for the effect to be the same. If the atomic group only surrounds the alternation (e.g., (?>a|b)*), the behavior will differ.

Example Comparison

Consider the following examples:

• (?:a|b)*+b and (?>(?:a|b)*)b will both fail to match the string b because the possessive quantifier or atomic group prevents the pattern from backtracking.
• In contrast, (?>a|b)*b will match b. The atomic group ensures that each alternation (a or b) doesn’t backtrack, but the outer greedy quantifier allows backtracking to match the final b.

Practical Tip for Conversion

When converting a regex from a flavor that supports possessive quantifiers to one that doesn’t, you can replace possessive quantifiers with atomic groups. For instance:

• Replace X*+ with (?>(X*)).
• Replace (?:a|b)*+ with (?>(?:a|b)*).

Using 3rd party tools can automate this conversion process and ensure compatibility across different regex flavors.