I had an issue where I needed to support
search and replace of certain quoted strings in various scripts which take the
form of $(VARNAME). There’s a minor issue with Java where if the replaced by
string contains certain characters where it will quit with an unhelpful message
like “IndexNotFound: Match group 5 not found” or something similar. This is how to
resolve that error, and the importance of escaping things.
The method Matcher.appendReplacement is used to search and replace in a
specialized fashion with Java regexes for instance we need to take some action
based on what the exact match was. The example shown on the javadoc is as
follows:
Patternp=Pattern.compile("cat");Matcherm=p.matcher("one cat two cats in the yard");StringBuffersb=newStringBuffer();while(m.find()){m.appendReplacement(sb,"dog");}m.appendTail(sb);System.out.println(sb.toString());
The above will replace all instances of “cat” with “dog”. However what if the need
is to replace “cat” by something with special characters like “$”. You’ll get an
exception like so:
Exception in thread "main" java.lang.IllegalArgumentException: Illegal group reference: group index is missing
at java.util.regex.Matcher.appendReplacement(Matcher.java:819)
at T.main(T.java:9)
This is all explained in the javadoc per the quote below:
Note that backslashes () and dollar signs ($) in the replacement string may cause
the results to be different than if it were being treated as a literal replacement string.
Dollar signs may be treated as references to captured subsequences as described above,
and backslashes are used to escape literal characters in the replacement string.
Luckily java provides a wonderful method called, Matcher.quoteReplacement that handles
the quoting for the replacement, and the output is exactly as expected when the string
replaced string uses that.