regularexpr.java

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package ranab.util;


/**
 * This is a simplified regular character mattching class. 
 * Supports *?^[]- pattern characters.
 * 
 * @author <a href="mailto:[e-mail]">Rana Bhattacharyya</a>
 */
public
class RegularExpr {
    
    private char[] mcPattern;
    
    /**
     * Constructor.
     * @param pattern regular expression
     */
    public RegularExpr(String pattern) {
        mcPattern = pattern.toCharArray();
    }
    
    /**
     * Compare string with a regular expression.
     */
    public boolean isMatch(String name) {
        return isMatch(name.toCharArray(), 0, 0);
    }
    
    
    /**
     * Is a match?
     */
    private boolean isMatch(char[] strName, int strIndex, int patternIndex) {
           
        while(true) {
            
            // no more pattern characters
            // if no more strName characters - return true
            if(patternIndex >= mcPattern.length) {
               return strIndex == strName.length;
            }
            
            char pc = mcPattern[patternIndex++];
            switch(pc) {
                
                // Match a single character in the range
                // If no more strName character - return false
                case '[' :
                
                    // no more string character - returns false
                    // example : pattern = ab[^c] and string = ab
                    if(strIndex >= strName.length) {
                        return false;
                    }
                    
                    char fc = strName[strIndex++];
                    char lastc = 0;
                    boolean bMatch = false; 
                    boolean bNegete = false;
                    boolean bFirst = true;
                    
                    while(true) {
                        
                        // single character match
                        // no more pattern character - error condition.
                        if(patternIndex>=mcPattern.length) {
                            return false;
                        }
                        pc = mcPattern[patternIndex++];
                        
                        // end character - break out the loop
                        // if end bracket is the first character - always a match.
                        // example pattern - [], [^]
                        if(pc == ']') {
                            if (bFirst) {
                                bMatch = true;
                            }
                            break;
                        }
                        
                        // if already matched - just read the rest till we get ']'.
                        if(bMatch) {
                            continue;
                        }
                        
                        // if the first character is the negete 
                        // character - inverse the matching condition
                        if((pc == '^') && bFirst) {
                           bNegete = true;
                           continue;
                        }
                        
                        bFirst = false;
                        
                        // '-' range check
                        if(pc == '-') {
                            
                            // pattern string is [a-  error condition.
                            if(patternIndex>=mcPattern.length) {
                                return false;
                            }
                            
                            // read the high range character and compare.
                            pc = mcPattern[patternIndex++];
                            bMatch = (fc>=lastc) && (fc<=pc);
                            if (bNegete) {
                                bMatch = !bMatch;
                            }
                            lastc = pc;
                        }
                        
                        // Single character match check. It might also be the
                        // low range character.
                        else {
                            lastc = pc;
                            bMatch = pc == fc;
                            if (bNegete) {
                                bMatch = !bMatch;
                            }
                        }
                    }
                    
                    // no match - return false.
                    if(!bMatch) {
                        return false;
                    }
                    break;
                    
                    
                // * - skip zero or more characters
                // No more patern character - return true    
                // Increment strIndex till the rest of the pattern matches.
                case '*' :
                
                    // no more string character remaining  - returns true 
                    if(patternIndex >= mcPattern.length) {
                        return true;
                    }
                    
                    // compare rest of the string
                    do {
                      if(isMatch(strName, strIndex++, patternIndex)) {
                          return true;
                      }  
                    }
                    while(strIndex < strName.length);
                    
                    // Example pattern is (a*b) and the string is (adfdc).
                    return false;
                
                    
                // ? - skip one character - increment strIndex.
                // If no more strName character - return false.    
                case '?' :
                
                    // already at the end - no more character - returns false
                    if(strIndex >= strName.length) {
                        return false;
                    }
                    strIndex++;
                    break;
                    

                // match character.    
                default:
                    
                    // already at the end - no match
                    if (strIndex >= strName.length) {
                        return false;
                    }
                    
                    // the characters are not equal - no match
                    if(strName[strIndex++] != pc) {
                        return false;
                    }    
                    break;
            }
        }
    }
    
}