When should I prefer regex over built-in string functions?

Question

Some say I should use regex whenever possible, others say I should use it at least as possible. Is there something like a "Perl Etiquette" about that matter or just TIMTOWTDI?

Answer 1

The level of complexity generally dictates whether I use a regex or not. Some of the questions I ask when deciding whether or not to use a regex are:

• Is there no built string function that handles this relatively easily?
• Do I need to capture substring groups?
• Do I need complex features like look behind or negative sets?
• Am I going to make use of character sets?
• Will using a regex make my code more readable?

If I answer yes to any of these, I generally use a regex.

Answer 2

I think a lot of the answers you got already are good. I want to address the etiquette part because I think there is some.

Summed up: if there is a robust parser available, use it instead of regular expressions; 100% of the time. Never recommend anything else to a novice. So–

Don'ts

• Don't split or match against commas for CSV, use Text::CSV/Text::CSV_XS.
• Don't write regexes against HTML or XML, use XML::LibXML, XML::Twig, HTML::TreeBuilder, HTML::TokeParser::Simple, et cetera.
• Don't write regexes for things that are trivial to split or unpack.

Dos

• Do use substr, index, and rindex where appropriate but recognize they can come off "unperly" so they are best used when benchmarking shows them superior to regular expressions; regexes can be surprisingly fast in many cases.
• Do use regular expressions when there is no good parser available and writing a Parse::RecDescent grammar is overkill, too much work, or will be too slow.
• Do use regular expressions for throw-away code like one-liners on well-known/predictable data including the HTML/CSV previously banned from regular expression use.
• Do be aware of alternatives for bigger problems like P::RecD, Parse::Yapp, and Marpa.
• Do keep your own council. Perl is supposed to be fun. Do whatever you like; just be prepared to get bashed if you complain when not following advice and it goes sideways. :P

Answer 3

I don't know of any "etiquette" about this.

Perl regex are highly optimized (that's one of the things the language is known for, although there are engines that are faster), and in the end, if your regex is so simple that it could be replaced by a string function, I don't believe that the regex will be any significantly less performant. If the problem you are trying to resolve is so time sensitive that you might look into other possibilities of optimization.

Another important aspect is readability. And I think that handling all string transformations through regex also add to this, insteas of mixing and matching different approaches.

Just my two cents.

Answer 4

Though I would classify this as too opinionated for SO, I'll give my point of view.

Use regex when the string is:

• "Too Dynamic" (The string could have a lot of variation to it, that making use of the string library(ies) would be cumbersome.
• "Contains patterns" if there is a genuine pattern to the string (and may be as simple as 1 character or a group of characters) this is where (i feel) regex excels.
• "Too Complex" If you find yourself declaring a whole function block just to do what a single pattern can do, I can see it being worthwhile just to use regex. (However, see "Too Complex" below, too).

Do not use regex to be:

• "Fast" Consider the overhead involved in spinning up a regex library over grabbing information directly from a string.
• "Too Complex" Good code isn't always short. If you begin making a huge pattern to circumvent several lines of code, that's fine, but keep in mind it's at the risk of readability. Coming back to that piece and trying to wrap your head around it again may not be worth just doing the plain-jane method.

Answer 5

I'd say, if you need more than one or two string function calls to do it, use a regex. ;)

Answer 6

Perl is a great language for regex. It honestly has one of the greatest parsers of any language, so that is why you see so many "use regex" answers. I am not sure what the aversion to regex is, however.

My answer would be: can you sum up the work in a single pattern easier than using the string function, or do you need to use multiple string functions versus a single regex? In either case, I would aim for regex. Otherwise, do what feels comfortable for you.

Answer 7

For things that are not too complex that the regex becomes bloated, affects the readability of code and cause performance issues. You can do it via a serious of steps, using builtin functions and other means. You may not have a cool single line regex, but your code will be readable and maintanable.

And also not too simple problems because, again, regexes are heavy weight and there are usually built-in functions that handled the simple scenarios.

It is going to depend on what you are going to do. Ofcourse, please don't use regex for parsing ( especially HTML etc. )