How to Match Any Character Across Multiple Lines in a Regular Expression

The Problem

Regular expressions (regex) are a powerful tool used for pattern matching in strings. However, they have a default behavior where they do not match newline characters by default. This can be a problem when you want to match a pattern that spans multiple lines. For example, consider the following scenario:

(.*)<FooBar>

This regex will match:

abcde<FooBar>

But it won't match:

abcde
fghij<FooBar>

Solution 1: Using the Dot-All Modifier

In order to match any character across multiple lines, you can use the (?s) modifier, also known as the 'dot-all' modifier. This modifier makes the dot character (.) match any character, including newline characters. The updated regex would look like this:

(?s)(.*)<FooBar>

This regex will now match:

abcde
fghij<FooBar>

Here, the (?s) modifier tells the regex engine to treat the dot as a 'dot-all' character. This means it will match any character, including newlines.

It's important to note that the 'dot-all' modifier affects the behavior of the dot character throughout the entire regex pattern, not just in one specific location.

Solution 2: Using the Single-Line Modifier

Another way to match any character across multiple lines is to use the (?m) modifier, also known as the 'single-line' modifier. This modifier changes the behavior of the caret (^) and dollar sign ($) metacharacters, making them match the beginning and end of each line, rather than the entire string. The updated regex would look like this:

(?m)(.*)<FooBar>

This regex will now match:

abcde
fghij<FooBar>

Here, the (?m) modifier tells the regex engine to treat each line as a separate entity. This means the dot character will match any character on the same line, but not newline characters.

Combining Modifiers

If you need to match any character across multiple lines and treat each line separately, you can combine both the 'dot-all' and 'single-line' modifiers. The updated regex would look like this:

(?sm)(.*)<FooBar>

This regex will match:

abcde
fghij<FooBar>

Here, the (?s) modifier allows the dot character to match any character, including newlines, and the (?m) modifier treats each line as a separate entity.

Example

Let's consider an example where we want to extract all the text between opening and closing <details> tags, including newlines:

<details>
    <summary>Lorem ipsum</summary>
    <p>Dolor sit amet,<br>
    consectetur adipiscing elit.</p>
</details>

We can achieve this using the following regex:

<details>(.*?)</details>

By default, this regex won't match because the dot character doesn't match newlines. But by adding the 'dot-all' modifier, the regex becomes:

(?s)<details>(.*?)</details>

Now, the regex will match the entire block of text between the <details> tags, including newlines. The captured group in this case would be:

<summary>Lorem ipsum</summary>
    <p>Dolor sit amet,<br>
    consectetur adipiscing elit.</p>

Conclusion

Matching any character across multiple lines in a regular expression can be achieved by using the 'dot-all' or 'single-line' modifier. The 'dot-all' modifier allows the dot character to match any character, including newlines, while the 'single-line' modifier treats each line as a separate entity, allowing the dot character to match any character on the same line.

By combining both modifiers, you can match any character across multiple lines and treat each line separately.

Regular expressions are a powerful tool for pattern matching, and understanding how to match across multiple lines can greatly expand their usefulness.