Part 9 — A complete beginner’s guide to Computer Programming with Clojure: Regular Expressions (REGEX).
Regular Expressions, otherwise known as REGEX, are a method of applying formulas to manipulate text. REGEX allows you to search, and filter swathes of text and extract only the desired. It can also be used to test a character string for specific features. For example, is it a MAC address, a mobile phone number, or an ISBN number? REGEX is a hugely powerful tool and often applied to forms and databases, as it can help control both input and output.
In Part 8, we used a REGEX to control input for an ISBN number.
#”^[0–9xX\-]+$”The REGEX followed a pattern, #”filter” . The filter usually starts with a ^ to indicate the beginning of a line, and a $ for the end of a line. Square brackets [ ] contain the specific REGEX pattern. For our ISBN, we wanted all numbers and the letter X in both upper and lower case. In addition, we required a hyphen -. To ensure we got a literal hyphen, and not a subtraction command, we used an escape character. The escape character is provided by a backslash \.
An escape character is used to tell the computer to use the following character in a non-standard way. For example, the letter n is simply interpreted as the character n. However, if we escape the letter n with a backslash \, we now get a newline \n.
Returning to our REGEX explanation, we see just before the dollar $ sign, a +. The + symbol indicates one or more. Next, curly brackets are used to specify the number of instances for the previous pattern. For example, when we needed to create a year filter, we used {3}.
#”^[12][0–9]{3}$”For our Year pattern above, [0–9] is any number. If we wanted, instead of 0–9, we could also use \d to specify decimal whole numbers. So, [0–9]{3} is any three-digit number e.g. 496. Which can also be written as [\d]{3}.
Consider the following:
(re-find #”\d+” “abc12345def678”)“12345”
So what happened to “678”?
The re-find command looks for the first instance of a REGEX pattern. Hence, as soon as it found a sequence of numbers (as indicated by #”\d+”, which is the same as [0–9]+), it stopped and returned the result.
What if we want to find all the numbers, “12345” and “678”?
We can use the re-seq command. All re-seq does is create a lazy list. In Clojure programming, the term lazy infers the list, or sequence, is only available after a function has been applied. In our case, re-seq is the function. Because the sequence is created on-demand, this actual creation is called a realization. Some evaluations are infinite, like PI or a third (3.3333 recurring). However, if the evaluation produces a finite value, it is referred to as fully-realized.
(re-seq #”\d+” “abc12345def678”)(“12345” “678”)
As you can see, we have found all groups of numbers. However, what if we wanted to find all groups of numbers and collect them together as a single string. For example, “12345678”.
(apply str (re-seq #”\d+” “abc12345def678”))“12345678”
As already discussed in the last Post on Functions, we have used apply to gather our lazy list into a single string.
Let’s look at another REGEX command, re-matches.
(re-matches #”\d+” “12345”)“12345”(re-matches #”\d+” “12345a”)nil
Recall, re-find will find and match the first instance of the requested pattern. But re-seq will carry on and find them all.
Our new pattern, re-matches, will only match the exact pattern. Hence, nil when a non-number appeared in the string.
For those occasions where you require an exact match, consider re-matches.
We can also use wild cards for occasions when we only wish to search with part of a string or we wish to return more than one result e.g. doing a database search.
(re-matches #”Star.*” “Star Trek”)“Star Trek”
Here we used a full-stop . to indicate any character and an asterisk * for none or more occurrences.
But what if we are not sure about the case? Consider the following.
(re-matches #”(?i)STAR.*” “Star Trek”)“Star Trek”
Effectively, by placing (?i) in front of our REGEX search string we instigate a search that is not case sensitive. Can you see how useful that is? You can now use an abbreviated search term that ignores the case.
Applying REGEX
Recall from Part 8 the code we used to filter for an ISBN number. Notice, this code follows a pattern:
This if statement pattern can be used to solve a number of programming problems. In programming, this is known as Selection. Recall, a computer program can be created from only three elements: Sequence, Selection, and Repetition. Our pattern above is an example Selection. The if statement checks a condition and if it is true it executes the green line. If it is false, it executes the red line. Again, by replicating the pattern above, you have a powerful tool to control Form input and output, as well as database and file searches.
SUMMARY
Like all things related to computer programming, subjects such as REGEX command a whole army of literature, websites, and debate. A number of sites offer huge amounts of pre-written REGEX for just about any situation. Nevertheless, finding the ‘perfect’ REGEX can prove elusive. In reality, for the budding programmer, REGEX creation often involves much trial and error.
