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A regular expression is a sequence of characters that describes or matches a given amount of text. For example, the sequence
bob, considered as a regular expression, would match any occurance of the word “bob” inside of another text. The following is a rather rudimentary introduction to the basics of regular expressions. We could spend the entire semester studying regular expressions if we put our mind to it. Nevertheless, we’ll just have a basic introduction to them this week and learn more advanced technique as we explore different text processing applications over the course of the semester.
Regular expressions (referred to as “regex” for short) have both literal characters and meta characters. In
bob, all three characters are literal, i.e. the “b” wants to match a “b”, the “o” an “o”, etc. We might also have the regular expression:
In this case, the
^ is a meta character, i.e. it does not want to match the character “^”, but instead indicates the “beginning of a line.” In other words the regex above would find a match in:
bob goes to the park.
but would not find a match in:
jill and bob go to the park.
Using the above, we could come up with some quick examples:
^$— matches beginning of line followed by end of line, i.e. match any blank line!
ing\b— matches “ing” followed by a word boundary, i.e. any time “ing” appears at the end of a word!
Character Classes serve as a kind of or statement amongst individual characters and are denoted by characters enclosed in brackets, i.e.
[aeiou] means match any vowel. Using a
^ negates the character class, i.e.
[^aeiou] means match any character not a vowel (note this isn’t just limited to letters, it really means anything at all that is not an a, e, i, o, or u.) A hyphen indicates a range of characters, such as
Another key metacharacter is
|, meaning or. This is known as the concept of alternation.
John | Jon means match “John” or Jon”
Note: this regex could also be written as
Joh?n, meaning match “Jon” with an optional “h” between the “o” and “n.”
Parentheses can also be used to constrain the alternation. For example,
(212|646|917)\d* matches any sequence of zero or more digits preceded by 212, 646, or 917 (presumably to retrieve phone numbers with NYC area codes). Note this regular expression would need to be improved to take into consideration white spaces and/or punctuation.
Parentheses also serve the purpose of capturing groups for back-references. For example, examine the following regular expression:
The first part of the expression in parentheses reads:
\b([0-9A-Za-z]+) meaning match any “word” containing at least one or more letters/digits. The next part
\s+ means any sequence of at least one white space. The third part
\1 says match whatever you matched that was enclosed inside the first set of parentheses, i.e.
([0-9A-Za-z]+). So, thinking this over, what will this regular expression match in the following line?
This is really really super super duper duper fun. Fun!
One quick way you can test regular expressions is with Atom text’s “find”. Simply enable the regex option (indicated by the
.* button) after hitting ⌘F.
Aa option makes the regular expression case-insensitive. Let’s look at some other examples (special thanks to Friedl”s Mastering Regular Expressions).
Checking this regex against shiffman.net/index.html yields the following results:
Here’s another regular expression example that uses a back reference to matches any repeated words
(Note, in the above example, the metacharacter
\b means “word boundary”, i.e. the beginning or end of a word.) Without checking for a word boundary the regex
(\w+)\s+\1 would match “This is” for example.
Find/replace with regex in Sublime Text is also incredibly useful. For example, here’s how you might change all markdown urls of the format
[name](link) to html
RegExp object has several methods. The first thing you might try is the
test() function which returns
false depending on if the string passed in matches the regex. This can be used to validate an e-mail address for example.
String object also has several methods which accept regular expressions as input. For example
search() is a function that works similarly to
indexOf(). It looks for a regular expression match and returns the index of that match.
Probably the most useful regular expression method is
match() is a method of the
String object that returns the matches of a regular expression.
results now contains the following array:
If the regular expression includes capturing parenthese, the groups would also appear in the array. For example, let’s say you needed a regex to match a phone number a string.
The above isn’t necessarily the greatest phone number matching regex, but it’ll do for this example. One or more numbers followed by a dash or period followed by one or more numbers, a dash or period again, and one or more numbers. Let’s look at the resulting array.
Notice how the full phone number match appears as the first (index 0) element and the captured group (the area code) follows. You might notice, however, that there are three phone numbers in the original input String and yet
match() only matched the first one. In order to find all the matches, we’ll need to add several other steps.
Regular expressions can include flags that modify how the search operates. For example the flag
i is for case-insensitivity so that the regular expression
hello with the flag
i would match “hello”, “Hello”, “HELLO”, and “hElLO” (and other permutations). A flag is added after the second forward slash like so:
/hello/i. The global flag
g tells the regular expression that we want to search for all of the matches and not just the first one.
Let’s look at what happens when you run the same code now.
match() now returns all of the matches in original string as elements of the array. However, the captured group for the area codes is now lost. If there are multiple matches and you want to have captured groups, you have to instead use the
exec() method which is part of the
In the case of a single match, the
exec() function works identically to
exec() function, even with the global flag, will still return only the first match. However, if you call
exec() a second time with the same regex and input string, it will move on and return the results for the second match (or
null if there is no match.) You can therefore write a
while loop to keep checking until the result is null.
This could also be written with the following shorthand (The examples linked from here, however, use the longer-winded code for clarity.)
I can now revisit the week 2 discussion of p5’s
splitTokens() functions and look at how regular expressions work as a delimiter with the native
split() method. In this case, a string is split into an array of substrings beginning at each match of that regular expression. Here’s a simple example that quickyl counts the # of words (not perfect by any means).
words array now contains:
What if you, however, would like to include all the delimiters? To accomplish this, simply enclose your delimiters in capturing parentheses. With
var regex = /(\W+)/; therefore you’ll get the following result.
Running a search and replace is one of the more powerful things one can do with regular expressions. This can be accomplished with the String’s
replace() method. The method receives two arguments, a regex and a replacement string. Wherever there is a regex match, it is replaced with the string provided.
The result is:
Replace every time ze word "ze" appears with ze word ze.
We can also reference the matched text using a backreference to a captured group in the substitution string. A backreference to the first group is indicated as
$2 is the second, and so on and so forth.
The result is:
Dououblee thee vooweels
replace() function also allows you to pass a callback where you can write your own code and
return the text that replaces any given match of the regex. This allows enormous flexibility and power because the logic of how you replace text can be written with code rather than simply encoded into a string. Let’s say you have some text:
You can then call
replace() to search with a regular expression (the following matches any words 3-5 characters long), and pass in a function call as the second argument.
The callback will be executed multiple times, as many times as the regex matches. The callback will receive the matched text as an argument and replace it with whatever you
The original text now reads:
The above result is silly and nonsensical, but it shows the beginnings of what is possible when you can execute any logic and call any other functions (query an API?) inside a
You can try out some regex below as well as take a look at the code for making this regex tester. While this works here, if you really want to just mess around with regex in the browser I recommend RegExr: Learn, Build< Test Regex.
You can also explore this walk-through of JS regex functionality.