Regexp question: Trouble matching with backslash

[Prabh]

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

 

[GaryM]

(e-mail address removed) (Prabh) wrote in @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 .

 

[Chris Smith]

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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Chris Smith - Lead Software Developer/Technical Trainer
MindIQ Corporation