Position within a string — Regular Expressions (Regexp)

In this lesson, we'll learn about special characters that are used to clarify positions of characters in strings and substrings.

Let's look at the following example:

/java/

java

Here, the word java matches the string java. This is a simple condition. It's important to remember that regular expressions don't look for matches in strings, but in substrings. If you search in a string that contains characters besides the ones you're looking for, the check may show that the strings match, even though that isn't what you want:

/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 string before the characters you're looking for, only the characters that are at the beginning of the string will match:

/^java/

java ruby clojurescript javascript

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

/java/

java ruby clojurescript javascript

A character that represents the end of a string

This character is $. Here is an example without using this character, with two matches:

/script/

java ruby clojurescript javascript

If we use $, then there's only one match, at the end of the string

/script$/

java ruby clojurescript javascript

Searching for other characters relative to their word boundaries

Suppose we only need to find instances of a, that are 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, that are not at the end of a word:

/a\B/

java ruby clojurescript javascript

If you put \b in front of the character we're searching for, the characters at the beginning of the word will be found:

/\bj/

java ruby clojurescript javascript

Using inverting again:

/\Bj/

java ruby clojurescriptj javascript

For clarity, if we compare with the previous example, we added another j after the clojurescript in our string. Now let's find only instances of j that are neither at the beginning nor at the end of a word:

/\Bj\B/

java ruby clojurescriptj javascript

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