[ruby-core:94944] [Ruby master Bug#16158] "st" Character Sequence In Regex Look-Behind Causes Illegal Pattern Error When Combined With POSIX Bracket Expressions And Case Insensitivity Flag

Issue #16158 has been updated by duerst (Martin Dst).


Some more information: The onigmo documentation says (https://github.com/k-takata/Onigmo/blob/master/doc/RE#L270):
```
                     Subexp of look-behind must be fixed-width.
                     But top-level alternatives can be of various lengths.
                     ex. (?<=a|bc) is OK. (?<=aaa(?:b|cd)) is not allowed.
```

Now what onigmo does internally is apparently that it considers the st ligature as case equivalent to upper-case ST, which is again case equivalent to lowercase st. You can see that as follows:

```
$ ruby -ve 'puts(/\uFB06/i =~ "most")'
ruby 2.7.0dev (2019-07-06T03:43:38Z trunk f296c260ef) [x86_64-cygwin]
2
```

The st ligature is a single character, so its length is 1, but the length of ST and st is 2. So with the //i option, st seems to no longer be fixed width, and that's why onigmo refuses to deal with this and produces an error. So in some way, this is as per spec, although it's surprising and annoying.



----------------------------------------
Bug #16158: "st" Character Sequence In Regex Look-Behind Causes Illegal Pattern Error When Combined With POSIX Bracket Expressions And Case Insensitivity Flag
https://bugs.ruby-lang.org/issues/16158#change-81561

* Author: michaeltomko (Michael Tomko)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-09-11 master 146677a1e7) [x86_64-openbsd6.5]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
*This is my first Ruby bug submission. Please let me know if there is anything else that I can provide that would be helpful. Thanks for your time!*

I've tried just about as many combinations as I can think of and I have been able to narrow down the issue to the following components being present in a regular expression.

* The character sequence "st" either preceded by any characters OR being a part of a top-level alternation inside of a look-behind. The issue occurs with both positive and negative look-behinds. ex: `(?<!Costa)` or `(?<!Bob|Sally|Stan)` or `(?<= st)`
* Case insensitivity either being set globally or inside of the regex with `(?i)` preceding the look-behind.
* Any curly-style POSIX bracket expression included anywhere in the regex. ex: `\p{Space}` or `\p{L}`

Here are some examples of the error. I have tested this on 2.5.0 locally and [on 2.5.3 with Rubular] (https://rubular.com/r/jnr98E9JfAZJIQ).

```
2.5.0 :044 > pat = /(?<!a st)\p{Space}/i
Traceback (most recent call last):
SyntaxError ((irb):44: invalid pattern in look-behind: /(?<!a st)\p{Space}/i)

2.5.0 :047 > pat = /(?i)(?<!a st)\p{Space}/
Traceback (most recent call last):
SyntaxError ((irb):47: invalid pattern in look-behind: /(?i)(?<!a st)\p{Space}/)

2.5.0 :016 > pat = /(?<!Costa)Mesa(\p{Space}|\p{Punct})+(AZ|Arizona)/i
Traceback (most recent call last):
SyntaxError ((irb):16: invalid pattern in look-behind: /(?<=Costa)Mesa(\p{Space}|\p{Punct})+(AZ|Arizona)/i)
```

My expectation would be that this regular expression would compile as written, as it does in JRuby and in MacOS regex testing apps like Patterns or Reggy.

It does compile as expected if the case insensitivity flag is removed or instantiated after the look-behind, if the "st" character sequence is first in the look-behind and not apart of an alternation, or if different types of operators are substituted for the POSIX bracket expressions.


```
2.5.0 :007 > pat = /((?<!Cosa)Mesa|Arlington(?=([:space:]|[:punct:])+(AZ|Arizona)))/
 => /((?<!Cosa)Mesa|Arlington(?=(\p{Space}|\p{Punct})+(AZ|Arizona)))/

2.5.0 :008 > pat = /((?<!Cosa)Mesa|Arlington(?=([:space:]|[:punct:])+(AZ|Arizona)))/i
 => /((?<!Cosa)Mesa|Arlington(?=([:space:]|[:punct:])+(AZ|Arizona)))/i

2.5.0 :009 > pat = /((?<!Cosa)Mesa|Arlington(?=([:space:]|[:punct:])+(AZ|Arizona)))/i
 => /((?<!Cosa)Mesa|Arlington(?=(\s|\W)+(AZ|Arizona)))/i

2.5.0 :056 > pat = /(?<!a st)(?i)(?<!juice)\p{Space}/
 => /(?<!a st)(?i)(?<!juice)\p{Space}/

2.5.0 :058 > pat = /(?<!a st)(?i)(?<!stark)\p{Space}/
 => /(?<!a st)(?i)(?<!stark)\p{Space}/
```



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