4 ms·

And yet regular expressions are a part of most programming languages as a library or built-in syntax. Why is that?

by icsa 4y ago

bear8642 4y ago
Probably due to they're a great notation for the problem area which for regex is concisely describing text patterns. For example 'a*b' is any number of 'a's followed by 'b'. How else would you concisely state that?
- LAC-Tech 4y ago
  How else would you concisely state that? Presumably people who hate array languages think all 3 character regexes should instead be big nested loops, so they are "readable".
- samatman 4y ago
  Regexes in the Unix tradition are a user interface as much as a programming language. Not that there's a sharp distinction, but it's almost a trite observation that regexes per se shine for ad hoc string searching but show their weakness when they start becoming parts of programs. When writing a program, I prefer to use a PEG, giving the less compact notation `'a'* 'b'` but also letting me say `'a'* b` and define b as its own rule, including recursion for the useful cases. It helps that it's more powerful, being little more than a formalization of the post-regular strategies used in Perl-style 'regular' expressions while embracing recursion. For '/' in vim, grep, wherever? Yeah regex is fine, that's what it was designed for.
- IshKebab 4y ago
  I can't remember the names but I've seen at least two alternative syntaxes recently that are a lot more readable. At least one of them fixed the issue of regex mixing up control in-band with data. So your example would be something like "a"* "b" Much more readable and less error-prone.
  - bear8642 4y ago
    > the issue of regex mixing up control in-band with data Could you explain this? I don't quite understand what the problem is. Do you mean something like sed's regex substitute command?
    - woojoo666 4y ago
      I believe they mean the operators and operands are all mixed up, eg in `ab`, and this makes it so you have to escape all sorts of characters, but if you split it into `"a" "b"` then the separation is clear
    - IshKebab 4y ago
      I mean it isn't clear whether a character is a control character (* + ? [ ] - etc) or a literal character because they're all mixed up. The rules about which is which are too complex, extensive and varying. If you use syntax like "a"* "b" then it's really obvious - the stuff in quotes is literal text, everything else is control. Lots of formats make the same mistake, e.g. YAML.
IshKebab 4y ago
They're very quick to write and they (in appropriate cases) would be quite difficult to implement otherwise (tedious state machine stuff). They are still massively overused though. Using a regex at all is a huge red flag. Sometimes they are appropriate, but not in 90% of cases in my experience. Anyway I'm not sure the same is true for K. At least for the given example the for loop was not exactly difficult to write.

And yet regular expressions are a part of most programming languages as a library or built-in syntax. Why is that?

by icsa 4y ago

And yet regular expressions are a part of most programming languages as a library or built-in syntax. Why is that?

Probably due to they're a great notation for the problem area which for regex is concisely describing text patterns. For example 'a*b' is any number of 'a's followed by 'b'. How else would you concisely state that?

LAC-Tech 4y ago
How else would you concisely state that? Presumably people who hate array languages think all 3 character regexes should instead be big nested loops, so they are "readable".
samatman 4y ago
Regexes in the Unix tradition are a user interface as much as a programming language. Not that there's a sharp distinction, but it's almost a trite observation that regexes per se shine for ad hoc string searching but show their weakness when they start becoming parts of programs. When writing a program, I prefer to use a PEG, giving the less compact notation `'a'* 'b'` but also letting me say `'a'* b` and define b as its own rule, including recursion for the useful cases. It helps that it's more powerful, being little more than a formalization of the post-regular strategies used in Perl-style 'regular' expressions while embracing recursion. For '/' in vim, grep, wherever? Yeah regex is fine, that's what it was designed for.
IshKebab 4y ago
I can't remember the names but I've seen at least two alternative syntaxes recently that are a lot more readable. At least one of them fixed the issue of regex mixing up control in-band with data. So your example would be something like "a"* "b" Much more readable and less error-prone.
- bear8642 4y ago
  > the issue of regex mixing up control in-band with data Could you explain this? I don't quite understand what the problem is. Do you mean something like sed's regex substitute command?
  - woojoo666 4y ago
    I believe they mean the operators and operands are all mixed up, eg in `ab`, and this makes it so you have to escape all sorts of characters, but if you split it into `"a" "b"` then the separation is clear
  - IshKebab 4y ago
    I mean it isn't clear whether a character is a control character (* + ? [ ] - etc) or a literal character because they're all mixed up. The rules about which is which are too complex, extensive and varying. If you use syntax like "a"* "b" then it's really obvious - the stuff in quotes is literal text, everything else is control. Lots of formats make the same mistake, e.g. YAML.

IshKebab 4y ago

They're very quick to write and they (in appropriate cases) would be quite difficult to implement otherwise (tedious state machine stuff). They are still massively overused though. Using a regex at all is a huge red flag. Sometimes they are appropriate, but not in 90% of cases in my experience. Anyway I'm not sure the same is true for K. At least for the given example the for loop was not exactly difficult to write.