Regexp question: Trouble matching with backslash

Hello all,
I traverse through a file, line-by-line and try to change the path in
a string which holds path, e.g., if the string is, "File is at
C:\AppFolder\Folder1\Foo.txt", I'm trying to change Folder1 to
Folder2, i.e.,

From: "C:\AppFolder\Folder1\Foo.txt"
To:   "C:\AppFolder\Folder2\Foo.txt"

Trouble is, I cant even check to see if the string has a match to
"C:\AppFolder\Folder1....", I suspect because of the backslashes.

The relevant piece of code:
 ========================================
=============================
String getLine = "File is at: C:\AppFolder\Folder1\Foo.txt" ;
PatternCompiler compiler = new Perl5Compiler() ;
Pattern pattern = compiler.compile(getLine) ;
PatternMatcher matcher = new Perl5Matcher() ;

if ( matcher.matches(getLine, pattern ) )
{
System.out.println("Its a match!") ;
} else {
System.out.println("It couldnt match!") ;
}

 ========================================
=============================

As you can see, I am basically matching a string against itself, just
to get going in the java regex learning process. I cant even match a
string to itself much less do a substitution on it.

Could some one give me some pointers on how to match and substitute
Folder1 with Folder2 ?

Thanks for your time,
Prabh

Prab_kar@hotmail.com (Prabh) wrote in news:e7774537.0405121404.739cef03
@posting.google.com:

>
> Could some one give me some pointers on how to match and substitute
> Folder1 with Folder2 ?
>

The problem is how both java and regex interpret the "\" character.

Try creating a pattern like this and matching it:

"C:\\\AppFolder\\\Folder2\\\Foo.txt"

Or just change them to "/"'s :).

Prabh wrote:
> The relevant piece of code:
>  ========================================
=============================
>  String getLine = "File is at: C:\AppFolder\Folder1\Foo.txt" ;
>  PatternCompiler compiler = new Perl5Compiler() ;
>  Pattern pattern = compiler.compile(getLine) ;
>  PatternMatcher matcher = new Perl5Matcher() ;
>
>  if ( matcher.matches(getLine, pattern ) )
>  {
>      System.out.println("Its a match!") ;
>  } else {
>      System.out.println("It couldnt match!") ;
>  }
>
>  ========================================
=============================

1) That code won't compile.  Backslashes have to be escaped in Java
string literal syntax, making them appear as "\\".

2) With regular expressions, the backslash is a special character there
as well, so it would need to be escaped again.  That causes it to appear
as "\\\\" (which, yes, is a little confusing at first, but is a logical
extension of the Java and Java-regexp languages).

As an extrapolation on #2, you'd be well-advised to avoid using *any*
regular expression for matching that wasn't written explicitly as a
regular expression.  Using an arbitrary String as a regexp is gambling
that the string won't contain characters with special meaning, and
that's not a safe bet.  Regular expressions, then, are useless unless:
1) you know the pattern at compile-time, or 2) your user knows what a
regular expression is, knows the Java-specific dialect of Perl regexp
language, and expects to be entering a regular expression at that point
in the application.  (And this advice even applies to places you might
expect, such as the replacement string in String.replaceAll and the
like; though not strictly a regular expression, it does interpret some
special characters, and hence carries the same set of restrictions.)

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