Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Regular Expressions – Mastering Lookahead and Lookbehind

rexegg.com

Post reply on HN
Print view View original

31–40 of 85 posts

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#31

Post by lifthrasiir »

asicsp wrote: post #27
lifthrasiir wrote: post #25

Earlier quoted context omitted.

Which is a fancy way to say `grep -iv '^c'`. EDIT: Oh, I missed that the input was a single line. I personally feel that control verbs are bad additions to the regexp, even though I do know that it is not a big addition to the regexp engine itself (e.g. naturally extended from posesssive quantifiers like `a++` or atomic groups `(?>foo)`). Most uses of such verbs can be expressed with combined parsers and simpler rege…

sorry, it is not same as `grep -iv '^c'` the `-o` option allows to output only matching portion, the regex is meant to extract all words other than those starting with 'c' or 'C' here's hopefully better example $ # do something with words not surround by quotes $ echo 'I like "mango" and "guava"' | perl -pe 's/"[^"]+"(*SKIP)(*F)|\w+/\U$&/g' I LIKE "mango" AND "guava"

Oh, you are right. I missed that all words are in the same line. That said even the original article mentions that it only moves the captured group to the entire match; I am generally in a position to avoid all uses of control verbs, especially if it only costs one or probably two lines of the additional code that I can fully control and comprehend.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#32

Post by arkh »

TeMPOraL wrote: post #22
jstarfish wrote: post #5

Earlier quoted context omitted.

They're definitely useful, and I can cobble them together to get lots of otherwise tedious and complex parsing tasks done, but when I come back to them a week later I have no idea what the hell the pile of wingding vomit I wrote was supposed to do. I find myself writing simpler ones and tying them together with app code just for sanity's sake.

Some regex implementations allow for comments in the string; if your does not, you can probably make it work with concatenation, like: String pattern = "^https+" // match the protocol at the beginning + "([a-zA-Z])+" // match the machine name + ... Honestly, I use regular expressions because, even in such format expanded with comments, I haven't seen anything more readable after you get used to regex operators. I gue…

If you're using PCRE you should also make use of named patterns. It makes the expression easier to understand as you can reuse parts of it (a little like functions) and the matched patterns can be then used in your language with their name instead of their position. Decoupling the usage from the regexp so it is more robust.

http://www.rexegg.com/regex-capture.html#namedgroups

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#33

Post by dash2 »

jstarfish wrote: post #5
filipmandaric wrote: post #2

Most of the time I mention the topic of regular expressions to other developers, I usually hear self-critical commentary like "oh, I'm terrible at regex", and rarely anyone who loves them. I think they're great though, if you take the time to understand them. They're something like a Swiss Army knife for programming.

They're definitely useful, and I can cobble them together to get lots of otherwise tedious and complex parsing tasks done, but when I come back to them a week later I have no idea what the hell the pile of wingding vomit I wrote was supposed to do. I find myself writing simpler ones and tying them together with app code just for sanity's sake.

I wonder if Perl 6 regexes and grammars might show a way forward for more readable pattern matching.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#34

Post by wruza »

roel_v wrote: post #30
Retra wrote: post #7

Earlier quoted context omitted.

A programmer saying they are terrible at regex is like a mathematician saying they are terrible at algebra.

Regex is useful when you do lots of string processing, like in webdev. Outside of that, I've found uses to be very limited - certainly not worth the upfront time investment. (I mean, sure one can cobble together something that mostly works with a regex testing tool, but you need to either take a college automata course or work through the Friedl in detail to get a basic level of proficiency).

I’ve found otherwise. No single work day passes without me inspecting/converting/refactoring calls and complex expressions via regex. Tools define the way you think and create.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#36

Post by amelius »

qiqitori wrote: post #8
filipmandaric wrote: post #2

Most of the time I mention the topic of regular expressions to other developers, I usually hear self-critical commentary like "oh, I'm terrible at regex", and rarely anyone who loves them. I think they're great though, if you take the time to understand them. They're something like a Swiss Army knife for programming.

I think everyone who doesn't know regex should make learning regex a priority. (However, I find that lookahead and lookbehind in particular do not tend to come in handy very often. So maybe just make a mental note that this exists and then look it up when you need it.) Just learn the basics and maybe take a very quick look at the theory, finite automata (maybe the name puts people off, but its just a couple of circle…

But don't forget to point at the limitations. For example, you can't use regexps to match an arbitrary but equal number of nested opening and closing parentheses.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#37

Post by zaarn »

filipmandaric wrote: post #2

Most of the time I mention the topic of regular expressions to other developers, I usually hear self-critical commentary like "oh, I'm terrible at regex", and rarely anyone who loves them. I think they're great though, if you take the time to understand them. They're something like a Swiss Army knife for programming.

Regex's are awesome as a swiss army knife.

Though in my experience, outside some edge cases where the format never changes (like matching a domain name in a URL) a regex is hell to maintain when you come back 3 years later.

There is also always the fun of people trying (and failing) to use regex in emails.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#38

Post by egeozcan »

Someone1234 wrote: post #15
qiqitori wrote: post #8

Earlier quoted context omitted.

I think everyone who doesn't know regex should make learning regex a priority. (However, I find that lookahead and lookbehind in particular do not tend to come in handy very often. So maybe just make a mental note that this exists and then look it up when you need it.) Just learn the basics and maybe take a very quick look at the theory, finite automata (maybe the name puts people off, but its just a couple of circle…

Which version of RegEx? I've "learned" RegEx two or three times and then switched language/platform and had everything I previously learned no longer work reliably. You might think I am just talking about Microsoft's quirky implementation but even in the Linux-sphere it isn't consistent see: http://www.greenend.org.uk/rjk/tech/regexp.html You take a complex format string which was design to use the fewest characters…

You wouldn't be programming in Regex and for small things a google search for the platform quirks is usually faster than writing a parser, isn't it?

I'd agree with you if everything weren't so easy to look up.

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#39

Post by oftenwrong »

amelius wrote: post #36
qiqitori wrote: post #8

Earlier quoted context omitted.

I think everyone who doesn't know regex should make learning regex a priority. (However, I find that lookahead and lookbehind in particular do not tend to come in handy very often. So maybe just make a mental note that this exists and then look it up when you need it.) Just learn the basics and maybe take a very quick look at the theory, finite automata (maybe the name puts people off, but its just a couple of circle…

But don't forget to point at the limitations. For example, you can't use regexps to match an arbitrary but equal number of nested opening and closing parentheses.

regex engines like PCRE can:

    ^(\((?1)?\))$

Re: Regular Expressions – Mastering Lookahead and Lookbehind

#40

Post by reificator »

filipmandaric wrote: post #2

Most of the time I mention the topic of regular expressions to other developers, I usually hear self-critical commentary like "oh, I'm terrible at regex", and rarely anyone who loves them. I think they're great though, if you take the time to understand them. They're something like a Swiss Army knife for programming.

Regex is great for very simple text processing, and I tend to do a lot of that.

The most important parts to get comfortable with are:

* Capture groups and alternation

* Character sets

* Anchors (start and end of line)

* Common escape sequences (digit, word, whitespace)

* Repeat (any, one or more, n-m)

* Common flags (global, multi-line, case-insensitive)

If you need more than that, it's time to start evaluating other tools IMO. A lookahead here and there is okay, but I'd avoid them if possible.

The best way I found to learn regex was to take a set of inputs that I wanted to match, (and a set that I didn't) and play around on https://regex101.com/ until I got a pattern that did what I wanted. You'll very quickly start to learn the above bulletpoints, and before long you'll be able to write patterns without any reference.

If you find that your regexes are getting too large or unwieldy or difficult to understand (despite knowing the above bulletpoints) then you probably need a parser or some other more suitable tool.

Post reply on HN