Difference between String replace() and replaceAll()

java

regex-patterns

performance-optimization

best-practices

byAnton Shumikhin·Nov 16, 2024

replace() performs a simple literal replacement while replaceAll() leverages regular expressions (regex) for more sophisticated replacement scenarios.

For instance, with replace():

String result = "foo".replace("oo", "ood"); // Outputs: "food" (as in "food for thought")

And using replaceAll() (regex pattern used):

String result = "foo123".replaceAll("\\d+", "456"); // Outputs: "foo456" (as in "area code swap-out!")

The nuts and bolts

Literal vs Pattern replacement

With replace(), you target a direct, literal replacement of a char or CharSequence. This provides a speed advantage when performing simple text swaps. Contrastingly, replaceAll() offers greater flexibility, catering to mutations dictated by regex patterns.

Beware of misuse

Casually choosing replaceAll() over replace() when the latter is sufficient can dampen performance. Why? Because replaceAll() goes the extra mile to compile regex patterns — a process that can be heavy for scenarios devoid of pattern-based replacements.

Making sensible choices

Practical examples are invaluable. Transforming periods to slashes could not have been simpler with replace(".", "/"). In the realm of data sanitization, replaceAll("[^a-zA-Z0-9]", "") comes handy to scrub non-alphanumeric characters.

Bugs. What bugs?

Regular expressions harbor immense power, yet wield it wrongly and brace for sneaky bugs. For instance, replaceAll(".", "#") can unexpectedly replace every character because of the misunderstood regex wildcard ".". Instead, apply replaceAll("\\.", "#") to specifically target periods.

Addressing intricacies

Special characters? Special!

Characters such as . and $ are born special in regex. They're treated literally in replace(), while in replaceAll(), they enjoy their special status and need escaping. So to replace dollar signs, replaceAll("\\$", "dollar") is your code.

Partial replacement

Use replaceFirst() to mark the first match that aligns with the regex and change that alone. This comes handy when you need to tweak just the first instance of a recurring pattern.

Complexities, not complications

For those unaware of the regex world, replace() remains a safety net. It eschews the complexities of pattern matching and offers a straightforward substitution mechanism.

Decoding decision making

Performance metric

When dealing with large strings or a high frequency of method calls, replace() has the edge. It deploys a simpler mechanism that doesn't require regex, significantly boosting the performance compared replaceAll().

Code clarity

For developers following your scripts, replace() signifies a simple swap, and replaceAll() hints at a pattern-based logic. The right function adds to code readability.