refactor(api): replace regex string literals with Pattern instances for improved performance and readability (#5680)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Signed-off-by: Balázs Szücs <[email protected]>

app/proprietary/src/main/java/stirling/software/proprietary/security/controller/api/UIDataTessdataController.java

@@ -9,6 +9,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.nio.file.StandardCopyOption;
 import java.util.*;
+import java.util.regex.Pattern;
 
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
@@ -36,6 +37,7 @@ import stirling.software.common.configuration.RuntimePathConfig;
 @RequiredArgsConstructor
 public class UIDataTessdataController {
 
+    private static final Pattern INVALID_LANG_CHARS_PATTERN = Pattern.compile("[^A-Za-z0-9_+\\-]");
     private final RuntimePathConfig runtimePathConfig;
     private static volatile List<String> cachedRemoteTessdata = null;
     private static volatile long cachedRemoteTessdataExpiry = 0L;
@@ -88,7 +90,7 @@ public class UIDataTessdataController {
                 failed.add(language);
                 continue;
             }
-            String safeLang = language.replaceAll("[^A-Za-z0-9_+\\-]", "");
+            String safeLang = INVALID_LANG_CHARS_PATTERN.matcher(language).replaceAll("");
             if (!safeLang.equals(language)) {
                 failed.add(language);
                 continue;

app/proprietary/src/main/java/stirling/software/proprietary/security/service/TotpService.java

@@ -6,6 +6,7 @@ import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.SecureRandom;
 import java.time.Instant;
+import java.util.regex.Pattern;
 
 import javax.crypto.Mac;
 import javax.crypto.spec.SecretKeySpec;
@@ -33,6 +34,7 @@ public class TotpService {
     private static final String HMAC_ALGORITHM = "HmacSHA1";
     private static final String DEFAULT_ISSUER = "Stirling PDF";
     private static final SecureRandom SECURE_RANDOM = new SecureRandom();
+    private static final Pattern TOTP_CODE_PATTERN = Pattern.compile("\\d{6}");
 
     private final ApplicationProperties applicationProperties;
 
@@ -71,7 +73,7 @@ public class TotpService {
         }
 
         String normalizedCode = code.replace(" ", "");
-        if (!normalizedCode.matches("\\d{6}")) {
+        if (!TOTP_CODE_PATTERN.matcher(normalizedCode).matches()) {
             return null;
         }
 

app/proprietary/src/main/java/stirling/software/proprietary/service/SignatureService.java

@@ -10,6 +10,7 @@ import java.nio.file.StandardOpenOption;
 import java.util.ArrayList;
 import java.util.Base64;
 import java.util.List;
+import java.util.regex.Pattern;
 import java.util.stream.Stream;
 
 import org.springframework.stereotype.Service;
@@ -32,6 +33,7 @@ import stirling.software.proprietary.model.api.signature.SavedSignatureResponse;
 @Slf4j
 public class SignatureService implements PersonalSignatureServiceInterface {
 
+    private static final Pattern FILENAME_VALIDATION_PATTERN = Pattern.compile("^[a-zA-Z0-9_.-]+$");
     private final String SIGNATURE_BASE_PATH;
     private final String ALL_USERS_FOLDER = "ALL_USERS";
     private final ObjectMapper objectMapper = new ObjectMapper();
@@ -366,14 +368,14 @@ public class SignatureService implements PersonalSignatureServiceInterface {
         if (fileName.contains("..") || fileName.contains("/") || fileName.contains("\\")) {
             throw new IllegalArgumentException("Invalid filename");
         }
-        if (!fileName.matches("^[a-zA-Z0-9_.-]+$")) {
+        if (!FILENAME_VALIDATION_PATTERN.matcher(fileName).matches()) {
             throw new IllegalArgumentException("Filename contains invalid characters");
         }
     }
 
     private String validateAndNormalizeExtension(String extension) {
         String normalized = extension.toLowerCase().trim();
-        if (normalized.equals("png") || normalized.equals("jpg") || normalized.equals("jpeg")) {
+        if ("png".equals(normalized) || "jpg".equals(normalized) || "jpeg".equals(normalized)) {
             return normalized;
         }
         throw new IllegalArgumentException("Unsupported image extension: " + extension);