Regex for replacing part of a pattern

[Satya Maheshwari]

I have a requirement where i need to replace all '123' in a file by '456' except where it is prefixed and/or suffixed with any alphabet or number.

For e.g
123 becomes 456
/123 becomes /456
123/ becomes 456/
123a remains 123a
a123 remains a123
1234 remains 1234
4123 remains 4123

How should I do this?

[Ankit Garg]

Can you give some more details like what is the maximum size of the string that you are using. I mean can there be a string like this "/]123[{" or anything like that??

[Edit: added some help instead of just lectures]

[Mayur Somani]

[^a-zA-Z0-9]123[^a-zA-Z0-9]

{Thanks Ankit for hint }

[James Tharakan]

Mayur Somani wrote:

[^a-zA-Z0-9]123[^a-zA-Z0-9]

{Thanks Ankit for hint }

Did you try for the input "123"

[Satya Maheshwari]

Ankit Garg wrote:Can you give some more details like what is the maximum size of the string that you are using. I mean can there be a string like this "/]123[{" or anything like that??

[Edit: added some help instead of just lectures]

The string can be of any length. Not fixed. It may be like "123 /123 asfewf =123/ferwfew q1234re"

[Satya Maheshwari]

Mayur Somani wrote:

[^a-zA-Z0-9]123[^a-zA-Z0-9]

{Thanks Ankit for hint }

This would make /123/ to 456 instead of /456/ which is not what is required.

[Henry Wong]

Satya Maheshwari wrote:
This would make /123/ to 456 instead of /456/ which is not what is required.

Take a look at "zero width negative lookahead" and lookbehind, in the JavaDocs. These will match (or with negative, make sure to not match), yet, will not gobble the input that it is matching.

Henry

[Dhruva Mistry]

replacing type of function will do...
not knowing much about API methods for parsers but i think string api can help

logic can be

if

String s.equals(123) then get the index of "123" that will give position in a string where '1' among "123" is stored so you can replcae that way

else,

check::

String replaceFirst(String regex, String replacement)
Replaces the first substring of this string that matches the given regular expression with the given replacement.

at String 5.0 API: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html

[Dhruva Mistry]

please check

Matcher API in java.regex package there few replacing methods given