Regex is correct but java won't parse it ?

[News]

Hello all,

I want to create a logic class to evaluate simple logical epxressions and
print their truth table. I am using a regular epxression that looks for a
pattern commencing with a char and followed by any number of (operator char)
groups, (for the sake of simplicity only the AND operator "&" is included
till I get it working properly).

My regex is [a-b]([&][a-b])*. I know the regex is correct because I have
tested it using the regular expression demo from
www.regular-expressions.info .

Following is my code stripped to the essentials. As it stands this returns a
match for even misformed strings and I cannot see why !

import java.util.regex.*;
public class Logic {
public static void main(String[] args) {
StringBuffer strb = new StringBuffer();
for (int i = 0; i < args.length; i++) {
strb.append(args); //Add the command line arguments to String Buffer }
String str = strb.toString(); //Change to a string so Matcher can use it.
String regex = new String("[a-z]([&][a-z])*");
System.out.println(str); //Test print to ensure the string and regex are
correct
System.out.println(regex);
Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE |
Pattern.UNICODE_CASE);
Matcher m = p.matcher(regex);
if (m.find()) {
System.out.println("Matched"); }
else {
System.out.println("Not Matched"); }
}
}

Any ideas ? Thanks in advance !

 

[Joshua Cranmer]

if (m.find()) {

find() returns if there exists a substring that matches the expression.
For example, you regex will match "3453457a4234456456" because there is
an 'a' in the expression. What you want is match().

 

[Stefan Ram]

find() returns if there exists a substring that matches the expression.
For example, you regex will match "3453457a4234456456" because there is
an 'a' in the expression. What you want is match().

Possibly, you meant »matches()« - there seems to be no »match()« method
in the class »Matcher«:

http://download.java.net/jdk7/docs/api/java/util/regex/Matcher.html#matches()

 

[Joshua Cranmer]

Possibly, you meant »matches()« - there seems to be no »match()« method
in the class »Matcher«:

http://download.java.net/jdk7/docs/api/java/util/regex/Matcher.html#matches()

Too much JavaScript for me, then.

Alternatively, using the regex "^[a-z]([&+*-][a-z])*$" with find would
also work, provided that the string is only one line line long.

Interestingly enough, from the URL you provided, you seem to be using
JDK 7. What's different from 1.6 (so far)?

 

[Stefan Ram]

Interestingly enough, from the URL you provided, you seem to be using
JDK 7. What's different from 1.6 (so far)?

http://tech.puredanger.com/java7#roundup
http://tech.puredanger.com/java7

 

[Esmond Pitt]

I want to create a logic class to evaluate simple logical epxressions
and print their truth table. I am using a regular epxression that looks
for a pattern commencing with a char and followed by any number of
(operator char) groups, (for the sake of simplicity only the AND
operator "&" is included till I get it working properly).

Hold on. The minute you get to handling "|" as well as "&" you will
discover that this is not a regular-expression problem, it is a parsing
problem. You will need to implement operator precedence, and REs can't
do that.

 

[News]

Hi Esmond, Joshua and Stefan,

Thanks for pointing out to me the difference between .find() and .matches().
It's a big step closer but .matches() returns false unless I replace the
regex with the EXACT string I am seaching for, eg "[a-z]([&][a-z])*" is
replaced with "p&q" and I search on "p&q". I also tried .LookingAt() but
still don't get a match. I alos tried using the escape sequence \\& in the
regex but no difference.

Esmond, I will certainly watch out for precedence issues once I get this
simple case working ! Thanks again. Herer is my latest.

import java.util.regex.*;
public class Logic {
public static void main(String[] args) {
StringBuffer strb = new StringBuffer();
for (int i = 0; i < args.length; i++) {
strb.append(args); //Add the command line arguments to String Buffer
}
String str = strb.toString(); //Change to a string so Matcher can use it.
String regex = new String("[a-z]([&][a-z])*");
System.out.println(str); //Test print to ensure the string and regex are
correct
System.out.println(regex);
Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE |
Pattern.UNICODE_CASE);
Matcher m = p.matcher(regex);
if (m.matches())
{
System.out.println("Matched");
}
else
{
System.out.println("Not Matched");
} } }

which when run with "p&q&n" produces

p&q&n
[a-z]([&][a-z])*
Not Matched

Any ideas? There's a beer in it!

Wayne

 

[Roedy Green]

import java.util.regex.*;
public class Logic {
public static void main(String[] args) {
StringBuffer strb = new StringBuffer();
for (int i = 0; i < args.length; i++) {
strb.append(args); //Add the command line arguments to String Buffer }
String str = strb.toString(); //Change to a string so Matcher can use it.
String regex = new String("[a-z]([&][a-z])*");
System.out.println(str); //Test print to ensure the string and regex are
correct
System.out.println(regex);
Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE |
Pattern.UNICODE_CASE);
Matcher m = p.matcher(regex);
if (m.find()) {
System.out.println("Matched"); }
else {
System.out.println("Not Matched"); }
}

I tidied and commented your code. In doing so the primary error jumped
out.

import java.util.regex.*;
public class Logic
{
public static void main(String[] args)
{
final StringBuffer strb = new StringBuffer();
for ( int i = 0; i < args.length; i++ )
{
strb.append(args); //Add the command line arguments to
StrinhBuffer
}
final String str = strb.toString(); //Change to a string so
Matcher can use it.
// look for string of the form ---a&a&b&c---
final String regex = "[a-z]([&][a-z])*";
System.out.println("command line:" + str); //Test print to
ensure the string and regex are correct
System.out.println("regex:" + regex);
final Pattern p = Pattern.compile(regex,
Pattern.CASE_INSENSITIVE |
Pattern.UNICODE_CASE);
// scan command string, not the regex.
final Matcher m = p.matcher(str);
if ( m.find() )
{
System.out.println("Matched");
// add some more printout to see what was matched.
final int gc = m.groupCount();
// group 0 is the whole pattern matched,
// loops runs from from 0 to gc, not 0 to gc-1 as is
traditional.
for ( int i=0; i<=gc; i++ )
{
System.out.println( i + " : " + m.group( i ) );
}
}
else
{
System.out.println("Not Matched");
}
}
}

 

[Roedy Green]

My regex is [a-b]([&][a-b])*. I know the regex is correct because I have

tested it using the regular expression demo from

if by any chance you are trying the find &xxxx; entities, see
http://mindprod.com/products.html#ENTITIES for a canned solution.

 

[News]

Hey Roedy,

There is no ASCII symbol for smacking yourself in the forehead and kicking
the cat so all I can say is thanks !!

My regex is [a-b]([&][a-b])*. I know the regex is correct because I have
tested it using the regular expression demo from

if by any chance you are trying the find &xxxx; entities, see
http://mindprod.com/products.html#ENTITIES for a canned solution.

 

[Roedy Green]

There is no ASCII symbol for smacking yourself in the forehead and kicking

the cat so all I can say is thanks!!

At http://mindprod.com/jgloss/regex.html are some code snippets for
doing the usual things with regexes. If you start with them, then
modify the code, you will likely avoid errors like the one that threw
you.

 

[Andrew Thompson]

News wrote:
...

There is no ASCII symbol for smacking yourself in the forehead and kicking
the cat ...

Did the cat write the code?

If not, I suggest it more appropriate, if no less violent,
to kick the ..entity or being that wrote the code.

 

[Lew]

...

Did the cat write the code?

If not, I suggest it more appropriate, if no less violent,
to kick the ..entity or being that wrote the code.

Maybe they meant "cat" in the beatnik sense, that is, they are going to kick
the "cat" that wrote it.

 

[Esmond Pitt]

Esmond, I will certainly watch out for precedence issues once I get this
simple case working !

Why would you bother to get it working when REs can't do it? You need to
build a tokenizer and a parser.

 

[Wayne McDermott]

Why would you bother to get it working when REs can't do it? You need to
build a tokenizer and a parser.

Howdy Esmond,

The StringTokenizer documentation actually recommends regular expressions be
used instead ! See
http://java.sun.com/j2se/1.4.2/docs/api/java/util/StringTokenizer.html

Cheers,

Wayne

 

[Chris Dollin]

Howdy Esmond,

The StringTokenizer documentation actually recommends regular expressions be
used instead ! See
http://java.sun.com/j2se/1.4.2/docs/api/java/util/StringTokenizer.html

Not a StringTokenizer; a tokeniser, aka lexer, aka lexical analyser, that
recognises tokens in the language, not just sequences separated by some
character.

If you're going to parse logical expressions, you will very soon go past
the stage where regular expressions can do the job, since you'll want
to tackle operators with different precedences, and brackets. It is
DEAD EASY to write a parser for simple expressions once you have the
tokens.

[You can use REs to recognise the tokens relatively easily.]

 

[Martin Gregorie]

Not a StringTokenizer; a tokeniser, aka lexer, aka lexical analyser, that
recognises tokens in the language, not just sequences separated by some
character.

If you're going to parse logical expressions, you will very soon go past
the stage where regular expressions can do the job, since you'll want
to tackle operators with different precedences, and brackets. It is
DEAD EASY to write a parser for simple expressions once you have the
tokens.

Its even easier to use something like Coco/R, which takes a single input
file and generates a Scanner (tokenizer) and a Parser class from it.
Even better, the frameworks for these classes are external text files,
so you can modify them. For instance, I needed a Scanner that would
accept a string to be processed - there was no constructor that would do
that but adding one was simple enough. As you'd hope, the Java version
of Coco/R is written in Java.

 

[Roedy Green]

Why would you bother to get it working when REs can't do it? You need to

build a tokenizer and a parser.

see http://mindprod.com/jgloss/parser.html

 

[bsgama]

in this line Matcher m = p.matcher(regex);, you shoud pass de str, not
the regex!



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