Need better Possessive Quantifier example

[jridgewell]

The current "prevents backtracking" is a bit ambiguous, since it could desugar into multiple similar regexs.

Given /^(?:a|.b)++$/ (purposefully using ?:, so we can ignore capturing groups for the moment):

1. ^(?>a|.b)+$/
2. /^(?>(?:a|.b)+)$/ (this one is correct)
3. /^(?>(?>a|.b)+)$/

This is explained in more detail in the "Using Atomic Grouping Instead of Possessive Quantifiers" section of https://www.regular-expressions.info/possessive.html.

[jridgewell]

Here are a few examples. I've modified the regex to demonstrate the issue, now using ^(?:a|.)++E$

• Original
• #1 desugaring (+ outside atomic)
• #2 correct desugaring (+ inside atomic)
• #3 desugaring (two atomics)
• ES5 positive lookahead desguaring

Note #2 is correct (and the ES5 desguaring).

#1's + outside atomic is easily shown as incorrect. This is because the + is keeping the backtracking. The atomic groups (.) branch will eat every char, but because the + is rematching the atomic group over and over, it still gets the backtracking info. Once (.) eats the E char, we exit the + loop. But now, we still need an E. The + will backtrack one time, retry, and now it matches.

It's a bit more difficult to demonstrate why #3's two atomics desguaring is incorrect. It's because the (?:a|.)+ should be allowed to backtrack as needed to complete. I'll keep trying to come up with a demo for this.

[jridgewell]

Actually, I don't think it's possible to construct a case where #3 is observably different than the correct desugaring, it's just more characters to parse.

[Keyacom]

I have a good application for the + possessive modifier: limited detecting of syntactically invalid HTML.

Since it's not yet available in ECMAScript, here's the PHP code for it:

<?php
# Requires PCRE 4.0 or newer!

$invalidHTMLre = '#</?[^\0- !-@[-`{-\x7f][^\0- !-,:-@[-`{-\x7f]*+(?!>)#gm';

$htmlToAlter = '<div></div><input></input';

echo htmlentities( preg_replace( $invalidHTMLre, '', $htmlToAlter ) );
# htmlentities escapes HTML into plain text
?>

This will output <div></div><input>. To see this regex in action: https://regex101.com/r/Hrw1SE/3