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Repeated Regex in Loop

Rule details
Rule ID repeated-regex-in-loop
Severity WARNING — likely performance problem
Confidence HIGH — structurally proven
Category Loop amplifiers
Complexity O(n·compile) → O(n)

Description

Detects regex pattern compilation inside loops. Compiling a regular expression is expensive — doing it on every iteration wastes CPU cycles when the compiled pattern could be reused.

Typical

for (String transactionRef : transactionRefs) {
    Pattern pattern = Pattern.compile("INV-\\d{4}-\\d{6}"); // Recompiled every iteration
    Matcher m = pattern.matcher(transactionRef);
    if (m.find()) {
        invoiceRefs.add(m.group());
    }
}

transactionRefs.forEach(ref => {
    const pattern = new RegExp('INV-\\d{4}-\\d{6}'); // Recompiled every iteration
    const match = ref.match(pattern);
    if (match) invoiceRefs.push(match[0]);
});

After

Pattern pattern = Pattern.compile("INV-\\d{4}-\\d{6}");
for (String transactionRef : transactionRefs) {
    Matcher m = pattern.matcher(transactionRef);
    if (m.find()) {
        invoiceRefs.add(m.group());
    }
}

const pattern = /INV-\d{4}-\d{6}/;
transactionRefs.forEach(ref => {
    const match = ref.match(pattern);
    if (match) invoiceRefs.push(match[0]);
});

Suggestion

Compile the pattern once outside the loop and reuse the compiled Pattern