Regular expressions are used to match certain patterns of strings within other strings.
They can be a useful tool for extracting patterns rather than exact strings, for example: telephone numbers (sequences of numbers of a specific length,) street numbers or email addresses.
var str = "how much wood would a woodchuck chuck if a woodchuck could chuck wood";
var regex = /wood/;
If we want to know whether the string "wood" appears in our larger string str we
could do the following
if (regex.test(str)) {
console.log("we found 'wood' in the string!");
}
=> "we found 'wood' in the string!"
To see the actual matches we found in the string, we can use the match method
to find all matches available:
var matches = str.match(regex);
console.log(matches);
=> ["wood"]
Note that this only returned one match, even though the word "wood" appears several
times in our original string. In order to find all individual instances of wood, we need
to add the global flag, which we can do by adding a g to the end of our expression:
regex = /wood/g;
console.log(str.match(regex));
=> ["wood", "wood", "wood", "wood"]
Now, note that two of those matches actually belonged to the word "woodchuck", which was not a part of our results. If we wanted to extend our regular expression to match both we could do so in a few ways:
regex = /wood.*?\b/g;
console.log(str.match(regex));
=> ["wood", "woodchuck", "woodchuck", "wood"]
In this regular expression we are matching everything that starts with the string "wood"
followed by 0 or more characters (.*?) until a word break (\b) occures.
Alternatively, we could also just search for both words:
regex = /woodchuck|wood/g;
console.log(str.match(regex));
=> ["wood", "woodchuck", "woodchuck", "wood"]
Note the order in which we did the last search. We used the word "woodchuch" before
the word "wood". If we were to run our expression like so: /wood|woodchuck/g, we would
end up with ["wood", "wood", "wood", "wood"] again because that search would be
"greedy".
If we wanted to replace the word "wood" in our original string, with the word "nun", we could do it like so:
regex = /wood/g;
var newstr = str.replace(regex, "nun");
console.log(newstr);
=> "how much nun would a nunchuck chuck if a nunchuck could chuck nun"
Probably not what you'd be going for, but you get our drift.
Extracting numbers from strings is a common task when looking for things like dollar amounts or any other numerical measurements that might be scattered about in the text. For example, if we wanted to extract the total amount of money spent on groceries from this message:
var message = "I bought a loaf of bread for $3.99, some milk for $2.49 and" +
"a box of chocolate cookies for $6.95";
we could define a regular expression that looks for dollar amounts by defining a pattern like so.
regex = /\$([0-9\.]+)\b/g;
this pattern looks for:
\$) to indicate the beginning of a price0-9) or the period character .. These can appear repeatedly (+). Note that we're not being particularly careful in making sure we only have one period in our string, for example.\b).If we wanted to find all the matches, we could use our string match function like so:
matches = message.match(regex);
console.log(matches);
=> ["$3.99", "$2.49", "$6.95"]
This is great! We have all our dollar amounts. While this gets us 90% there, we
can't really add them with those $ signs. To remove them, we can use our trusty
reduce function like so:
matches.reduce(function(sum, value) {
return sum + Number(value.slice(1));
}, 0);
=> 13.43
We've used a few special characters so far, like \b to indicate a word break. There
are a few others that might be useful to you:
\d - any number character. Equivalent to [0-9].\D - any non number character. Equivalent to [^0-9].\s - any single space character. This includes a single space, tab, line feed or
form feed.You can see a full list of all special characters here: MDN - Regular Expressions