Comment by developer1 | Hacker News Reader

Comment by developer1 | Hacker News Reader

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Why is everybody talking about ruby's regex implementation? ^, \A, $, \Z, \z are standard across many languages, based on PCRE syntax. This isn't a ruby problem; this is simply developers who are not experts with regular expressions not fully understanding how to write patterns with the tool they are using. There are no problems with regex implementations, only problems with regex use.

Anybody interested should have a read through http://pcre.org/pcre.txt. The syntax presented here is used in perl, php, ruby, python, and many others.

Also, nobody ever uses \A without the /m flag. You use ^, it has the same meaning unless you specifically add the /m flag to allow ^ to match at the beginning of any line rather than at the front of the string only. This distinction will only bite developers who just add flags like /msig for every regex, because again they don't understand exactly what every flag actually does.

> ^, \A, $, \Z, \z are standard across many languages, based on PCRE syntax.

Uhm... No? ^$ have always meant beginning and end of string, not line, unless you turn on a flag. Not by default. You can check PCRE's documentation if you don't believe me. And then, there's Ruby:

  irb(main):005:0> "foo\nbar".match /o$/
  => #<MatchData "o">

ruby has half multine mode by default, which causes the issue. Namely, /m only changes the behaviour of "." to match \n, but ^ and $ work the same.

(and incidentally, that would make changing it easier, you can just request that users specify a flag all the time and deprecate the one without).

Eww what the hell. I don't use ruby, and if I did I might very well - as a PCRE half-expert - fall into this trap based purely on the assumption that ruby was using PCRE. I just looked at their Regexp class, and it matches PCRE in most regards. The fact that /m makes . match newline \n is horrible - every PCRE-based implementation uses /s for that, whereas /m only affects ^ and $.

It still falls on the developer to understand the exact flavor of regex available in their language. And yet ruby is doing a disservice to anybody coming to their language with existing PCRE knowledge by having syntax that is almost an exact match to PCRE used in many languages... only to find out someday that it's not. Harsh.

Regular expressions are dangerously complex for validation code, it's too easy to overlook something or miss some context (like multiline mode, see also riffraff's comment).

So the problem is probably not developer knowledge (note that you got it wrong, too!), but rather that regexps are too hard to get right.

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