Position within a string — Regular Expressions (Regexp)

In this lesson, we will learn about special characters that can clarify the character's positions in strings and substrings.

Let us look at the following example:

/java/

java

Here, the word java matches the string java. It is a simple condition.

It is important to remember that regular expressions do not look for matches in strings but in substrings. If you search in a string containing characters besides the ones you look for, the check may show that the strings match, even though you did not want it:

/java/

asdfjava asdf

There are special characters to control character search in a string.

A character that represents the beginning of a line

If you put ^ in the search line before the characters you look for, only the characters at the beginning of strings will match:

/^java/

java ruby clojurescript javascript

If we remove ^, we will have two matches, including one in the last substring:

/java/

java ruby clojurescript javascript

A character representing the end of a string

This character is $. Here is an example without this character. It has two matches:

/script/

java ruby clojurescript javascript

If we use $, we will have only one match at the end of the string:

/script$/

java ruby clojurescript javascript

Searching for characters relative to their word boundaries

Suppose we only need to find instances of a at the end of a word. To do this, we type a\b in the string pattern:

/a\b/

java ruby clojurescript javascript

If we type \B, instead, we can invert the search and find all instances of a not at the end of a word:

/a\B/

java ruby clojurescript javascript

If you put \b in front of the character we search for, we will find the characters at the beginning of the word:

/\bj/

java ruby clojurescript javascript

Here we use inverting again:

/\Bj/

java ruby clojurescriptj javascript

For clarity, if we compare it with the previous example, we added another j after the clojurescript in our string.

Now we find only instances of j that are neither at the beginning nor the end of a word:

/\Bj\B/

java ruby clojurescriptj javascript

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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“