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Pattern.compile() should not be called in a loop JAVA-P0331

Performance
Critical

This method calls Pattern.compile() inside a loop with constant arguments. If this Pattern will be used several times, there's no reason to compile it on each loop iteration.

Pattern.compile() is an expensive method to run. This is because regexes are computationally intensive to generate. If regex compilation were to occur in a loop, the cumulative overhead from every iteration could cause an undesirable amount of lag.

Move this call outside of the loop or into a static final field.

Bad Practice

while (someCondition) {

    Pattern regex = Pattern.compile("(r|R)egex");

    // ...
}

Recommended

If the regex will be used multiple times over the course of the program, it is better to declare it separately, perhaps as a static field.

Pattern regex = Pattern.compile("(r|R)egex");

while (someCondition) {
    Matcher matches = regex.matcher(input);
    matches.find();
    // ...
}

If possible, turn the declaration into a static final field:

static final Pattern regex = Pattern.compile("(r|R)egex");

References