How can I match a regex in a switch statement?

Question

I am trying to match tokens with a switch statement in a function, and one of these tokens should be able to identify any string or any numbers defined by a regular expression in the code below.

Basically, is it possible to specify a regular expression for a case such as case "[a-z]+":

It’s clear that now I have it, my template is not available if I do not pass STRINGor NUMBERas an argument.

public Token analyzeToken(String token) {
      Token tokenType = null;     

      switch (token) {

         case "STRING":
            Pattern p = Pattern.compile("[a-z]+");
            Matcher m = p.matcher(token);
            if(m.matches()) {
               tokenType = Token.STRING;
               break;
            }
         case "NUMBER":
            Pattern p = Pattern.compile("[0-9]+");
            Matcher m = p.matcher(token);
            if(m.matches()) {
               tokenType = Token.NUMBER;
               break;

         case "(":
            tokenType = Token.LEFT_PAREN;
            break;
         case ")":
            tokenType = Token.RIGHT_PAREN;
            break;
         case ".":
            tokenType = Token.PERIOD;
            break;
         case ":":
            tokenType = Token.COLON;
            break;
         case ";":
            tokenType = Token.SEMICOLON;

         default:
            tokenType = TOKEN.UNKNOWN;
            break;

      }
   }

Answer 1

Do not do this in an instruction switch, do it in a conditional or better yet, in a loop:

private interface TokenMatcher {
    Token match(String s);
}
static List<TokenMatcher> matchers = new ArrayList<>();
static {
    final Pattern strPattern = Pattern.compile("[a-z]+");
    final Pattern numPattern = Pattern.compile("[0-9]+");
    matchers.add( new TokenMatcher {
        public Token match(String s) {
            Matcher m = strPattern.matcher(s);
            return m.matches() ? Token.STRING : null;
        }
    });
    matchers.add( new TokenMatcher {
        public Token match(String s) {
            Matcher m = numPattern.matcher(s);
            return m.matches() ? Token.NUMBER : null;
        }
    });
}

Now you can do this:

static Token match(String s) {
    for (TokenMatcher m : matchers) {
        Token t = m.match(s);
        if (t != null) {
            return t;
        }
    }
    return TOKEN.UNKNOWN;
}

for switch, matchers case switch. , matchers.

: , , :

private static final class TokenMatcher {
    private final Pattern p;
    private final Token t;
    public TokenMatcher(String pString, Token t) {
        p = Pattern.compile(pString);
        this.t = t;
    }
    public Token match(String s) {
        Matcher m = p.matcher(s);
        return m.matches() ? t: null;
    }
}

matchers :

matchers.add(new TokenMatcher("[a-z]+", Token.STRING));
matchers.add(new TokenMatcher("[0-9]+", Token.NUMBER));

Answer 2

@dasblinkenlight. .

public class TokenMatcher{

    private HashMap<String, Token> tokenMap = new HashMap<String, Token>();
    {
        tokenMap.put("[a-z]+", Token.STRING);
        tokenMap.put("[0-9]+", Token.NUMBER);
        tokenMap.put("\\(", Token.RIGHT_PARENT);
        tokenMap.put("\\)", Token.LEFT_PARENT);
        ...
    }


    public Token match(String s){
        for(String key : tokenMap.keySet()){
            Pattern pattern = Pattern.compile(key);
            Matcher matcher = pattern.matcher(s);
            if(matcher.matches()) return tokenMap.get(key);
        }
        return Token.UNKNOWN;
    }
}

: , , .

Answer 3

:

public Token analyzeToken(String token, String string) {
      Token tokenType = null;     
      switch (token) {
         case "STRING":
            Pattern p = Pattern.compile("[a-z]+");
            Matcher m = p.matcher(string); // match the string, not the token!
            if(m.matches()) {
               tokenType = Token.STRING;
               break;
            }

:

public Token analyzeToken(String regex, String string) {
            Pattern p = Pattern.compile(regex);
            Matcher m = p.matcher(string); // match the string, not the token!
            if(m.matches()) {
               // ...
            }

Answer 4

, , regex, . , .

enum PatternCase {
    pattern1("regexp1"),
    pattern2("regexp2"),
    pattern3("regexp3"),
    pattern4("regexp4", Pattern.CASE_INSENSITIVE),
    ;
    final Pattern pattern;
    PatternCase(String regExp) { pattern = Pattern.compile(regExp); }
    PatternCase(String regExp, int flags) { pattern = Pattern.compile(regExp, flags); }
}

Then you have a fully working switch with regex as the case statement.

        Matcher matcher = null;
        PatternCase matchedPatternCase = null;  // Or use some default value to avoid null checks.

        // Match only these patterns and match them in this effective order.
        for (PatternCase patternCase : new PatternCase[]{ PatternCase.pattern3, PatternCase.pattern1 }) {
            matcher = patternCase.pattern.matcher(text);
            if (matcher.find()) {
                matchedPatternCase = patternCase;
                break;
            }
        }

        if (matchedPatternCase != null) switch (matchedPatternCase) {
            case pattern1:
                System.out.println(matcher.group());
                break;
            case pattern3:
                do { System.out.println(matcher.group()); } while (matcher.find());
                break;
            default:
                break;
        }

If the value of your token is already listed, it can (or optionally) use the template directly in your Token enumeration and simply iterate over all the values ​​of the Token enumeration.