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Ask HN: How do I get started with regular expressions?
22 points by seanccox on March 27, 2013 | hide | past | favorite | 15 comments
I am not a programmer, I am a research investigator.

Thus, I have tinkered with code only on an as-needed basis, when I have had very specific problems that I knew a computer could do for me accurately and quickly – changing file names in a folder, performing repetitive editorial tasks, automatically filing downloaded sources with specific names. Most of these things were done in Applescript, and they are all good things for me, because the time they save me gets devoted to thinking that helps me build a case better. I can't hire an intern, because that's expensive, but my meager Applescripting I have done so far has made for a very reliable intern.

Recently, I wanted to write a script that would scan my documents, identify local currency figures, convert them to a global standard ($, €, or Bitcoin), and then insert them into my report copy. This seemed very reasonable to me, and I thought that it would be easy (since everything else I had done had been a simple matter of articulating the problem and finding the solution somewhere online). Time consuming, perhaps, but easy... like reading, copying, pasting, and then tinkering to customize it.

This was not easy.

Someone has suggested I try regular expressions to solve the problem. The thing is, that sentence doesn't mean anything to me, because I don't know where/how to begin, and I don't really know what regex is.

Where would you send someone like me to start learning regex?

Cheers, -s



Regular Expressions, by themselves, don't solve your problem. Regular expressions, implemented as part of a tool like sed or grep, or as part of a programming language like Perl or Java, can.

Regular expressions are basically just a (powerful) way to match patterns in text. If you've done "ls *.sh" in a directory to list only the .sh files, you've used something very much like a regular expression. But that's just working on filenames, tools like grep and sed can look inside a file and match patterns, and sed can then replace those patterns with whatever you supply.

If you just want to start off learning regexes, google "regex tutorial" and start playing around searching through files with grep or something.

For your specific case, you should be able to do what you want to do with sed[1], or with awk[2]. Or if you're more comfortable with a higher level language, you could go with anything that has regex support, which should be most mainstream languages.

[1]: http://en.wikipedia.org/wiki/Sed

[2]: http://en.wikipedia.org/wiki/AWK


The above is great advice. You will definitely want to play around with a tutorial, any tutorial, so you can grasp the basics of what regular expressions are, and how they are used. After you "get" what regular expressions are, you can work on implementing them in your code.

After you understand the basics, you can follow these steps that I use every time I need to program regular expressions.

1) Google a chart explaining the regular expression syntax for the language you are using (something like - applesscript regexp chart)

2) Create some test code to help refine your regular expression. Your code should have

* a - REGEX STRING - the regular expression value you are testing. You can store this value in a variable in your code, reference this value from a text file, or input it manually every time you run your script. I like to give myself access to this outside the code, so for a webpage have a form with an input field with this value. For scripting languages, make this a parameter of your script, so you can play with it quickly without having to change any code.

* b - STRING DATA - this is the value (or values) you will be comparing your regular expression against. You can store this in a variable, or reference this from a text file. Try and think of all cases that could potentially cause your script to break. For example, the currency symbols you are trying to replace, could they ever be used in ways where you would not want to replace them?

* c - RESULTS - you should output the results every time you run your script and keep tweaking it until you are satisfied with the output.

3) Implement your awesome REGEX in your code.


Great points!

On the regex front, I highly recommend starting with reference sheets[1].

There are also a great many testers out there. The best way to figure it out is by just testing your regex patterns against various strings. You'll eventually start to understand how each item works.

Don't get too frustrated with it! It can be tough to learn because of the syntax, but you'll get the hang of it once you practice.

[1]: http://www.regular-expressions.info/reference.html


Learn Regex the Hard Way

http://regex.learncodethehardway.org/book/


http://regular-expressions.info

"Mastering Regular Expressions" by Friedl is the ultimate resource, but for getting started that website above will let you learn the practical syntax.


The tutorial that helped me is "sed" based: http://www.gentoo.org/doc/en/articles/l-sed1.xml

Also, an online regex tester may be helpful: http://regexpal.com/


Here's a first link. It's a visual matching tool for your regex and targeted text + several saved regex expression by the community: http://gskinner.com/RegExr/


This is amazing! (and amazingly helpful with the Community tab to show examples!)


Start with the wikipedia page[1] and continue with the documentation of the regex engine you'll be using (there are differences between implementations).

[1] http://en.wikipedia.org/wiki/Regular_expression


This is another plug, but people have told me that they've found RegexOne (http://regexone.com) to be pretty useful in learning and practicing regular expressions interactively.


I liked Eric Lippert series of article on regular expressions http://blogs.msdn.com/b/ericlippert/archive/tags/regular+exp...


Wow, thanks for all this feedback. Once I get a chance to wade through these resources, I'll let ya'll know what I found the most useful. Thanks again.

Cheers, -s


Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.

- Jamie Zawinski


http://rubular.com/ is another way to get a feel for them.


http://regex.learncodethehardway.org/




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