Understanding Why String.replaceAll Doesn't Work with $ Symbols in Java Regex

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The Role of $ in Java Regular Expressions

In Java regular expressions, the $ symbol has a special function—it is used as an assertion for the end of a line. This means that within the context of a regex pattern, $ is not treated as a literal character but as a positional element indicating the line's end.

For example:

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The above code might not work as intended because $ is seen as an end-of-line marker, not as a literal dollar sign. Consequently, this can cause confusion and unexpected results.

The Double Role of $ in Replacement Strings

On the flip side, $ also plays a role in the replacement string within the replaceAll method. In replacement strings, $ is used to refer to capturing groups from the matching pattern. For example, $1 refers to the first capturing group, $2 to the second, and so on.

How to Escape the $ Symbol

To use the $ symbol literally, you need to escape it. Escaping ensures that the parser interprets $ as a regular character rather than a special symbol. Here’s how you can do it:

Escaping in the Regex Pattern
When the $ symbol is part of the pattern being matched, you need to escape it with a backslash: \$. Note that in Java strings, a backslash itself must be escaped with another backslash.

Example:

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Escaping in the Replacement String
If $ is part of the replacement text, it should be escaped with a single backslash.

Example:

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Summary

Whether you are defining a pattern or specifying a replacement, always remember the special handling of $ and use appropriate escaping to maintain the intended semantics of your string operations.