Greediness — Regular Expressions (Regexp)

In this lesson, we will observe greedy and lazy quantification.

We will start by writing a regular expression highlighting all the tags in our string. To do it, we should enter the opening and closing tag brackets: < and >. Here we put a character inside:

/<.*>/

<a href="https://www.yahoo.com">google</a>

There are two tags in the string – opening and closing. Although, we found a match for the whole string. Programmers call this behavior greedy. It means that the quantifier repeats as many times as possible.

In regular expressions, quantification is greedy by default, so we have captured the longest possible substring. The entire string corresponds to our regular expression; everything inside < and > counts as tag content.

To avoid greedy behavior, we can specify any character inside the tag instead of any character (except the closing one). In that case, we will process the two tags separately:

/<[^>]*>/

<a href="https://www.yahoo.com">google</a>

Despite the effectiveness of such a solution, it is only an escape from the problem. That way, we can avoid the consequences of greedy behavior but not the behavior itself.

To turn a greedy quantization into a lazy one, you have to use a special symbol ?:

/<.*?>/

<a href="https://www.yahoo.com">google</a>

We should remember that many special characters behave differently in regular expressions, depending on the place where they are. If ? were placed after the dot, it would be treated simply as a "no character or one character" quantifier:

/<.?>/

<p><a href=" www.yahoo.com ">google</a></p>

But when ? comes right after a quantifier, it's treated by regex as a special character that turns greedy quantifiers into lazy ones.

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About Hexlet learning process

• Article “How to Learn and Cope with Negative Thoughts“
• Article “Learning Traps“
• Article “Complex and simple programming tasks“