String Regex in Java - Puzzle

Question

I have the below String which contains multiple dates; something like this:

I was born on 8/11/1965. I need to change all the dates into a different format.

What is the best optimized way to achieve this ?

Note:

I have not asked someone to give me the code; only asking for suggestions; hence cant see any reason for the negative rating.

Answer 1

This is the complete algorithm.

Pattern ddpat = Pattern.compile( "\\d{1,2}/\\d{1,2}/\\d{4}" );
DateFormat dddf = new SimpleDateFormat("dd/MM/yyyy");
DateFormat wwdf = new SimpleDateFormat("EEEE, MMMM d yyyy");
String s = "I was born on 18/11/1965. and completed my grad on 9/10/1978 and so on";
StringBuilder sb = new StringBuilder();
Matcher ddmat = ddpat.matcher( s );
int offset = 0;
while( ddmat.find( offset ) ){
    int beg = ddmat.start();
    int end = ddmat.end();
    String dd = s.substring( beg, end );
    String ww = wwdf.format( dddf.parse( dd ) );
    sb.append( s.substring( offset, beg ) ).append( ww );
    offset = end;
}
sb.append( s.substring( offset ) );
System.out.println( sb.toString() );

To handle 99/99/9999 and similar, handle ParseException after dddf.parse( dd ) and replace with whatever you seem fit. Strings not matching the regex are hopeless, unless you can think of a regex matching "false dates". ;-)

Later

If you want to fish for several formats:

Pattern ddpat = Pattern.compile( "(\\d{1,2}/\\d{1,2}/\\d{4})|(\\d{4}-\\d{2}-\\d{2})" );
DateFormat[] dddf = new DateFormat[]{
    new SimpleDateFormat("dd/MM/yyyy"),
    new SimpleDateFormat("yyyy-MM-dd") };
DateFormat wwdf = new SimpleDateFormat("EEEE, MMMM d yyyy");
String s = "I was born on 18/11/1965. and completed my grad on 1978-10-09 and so on";
StringBuilder sb = new StringBuilder();
Matcher ddmat = ddpat.matcher( s );
int offset = 0;
while( ddmat.find( offset ) ){
    for( int ig = 1; ig <= ddmat.groupCount(); ig++ ){
    if( ddmat.group(ig) != null ){
        int beg = ddmat.start(ig);
        int end = ddmat.end(ig);
        String dd = s.substring( beg, end );
        String ww = wwdf.format( dddf[ig-1].parse( dd ) );
        sb.append( s.substring( offset, beg ) ).append( ww );
        offset = end;
        break;
    }
    }
}
sb.append( s.substring( offset ) );
System.out.println( sb.toString() );

Answer 2

If I understand your objective, the easiest approach I can think of is using two SimpleDateFormat instances. One for your input format, one for your output format. That is, something like -

String str = "8/11/1965";
DateFormat in = new SimpleDateFormat("dd/MM/yyyy");
DateFormat out = new SimpleDateFormat("EEEE, MMMM dd yyyy");
System.out.println(out.format(in.parse(str)));

Output is (because November 8, 1965 is a Monday not a Wednesday) -

Monday, November 08 1965

$ cal 11 1965
   November 1965      
Su Mo Tu We Th Fr Sa  
    1  2  3  4  5  6  
 7  8  9 10 11 12 13  
14 15 16 17 18 19 20  
21 22 23 24 25 26 27  
28 29 30