Question

From ant, we want to extract a line from an old /etc/shadow file, capturing the line for a specific user name, such as “manager”. This is part of a backup/restore operation. What we used previously was not specific enough, so it would match users like “mymanager”, so we tried to tighten it down by anchoring the start of the string to beginning of the line (typically “^”). This definitely did not work as we expected.

How can we anchor so that we get an exact match for a username? — answered below.

First attempt, which gave the wrong result if we had a user of “mymanager” in the /etc/shadow file copy:

<loadfile property="oldPasswords" srcFile="${backup.dir}/shadow"/>
<propertyregex property="manager.backup" input="${oldPasswords}"
    regexp="(manager\:.*)" select="\1" casesensitive="true" />

Second attempt, which failed because “^” is not interpreted in the normal regular expression way by default:

<loadfile property="oldPasswords" srcFile="${backup.dir}/shadow"/>
<propertyregex property="manager.backup" input="${oldPasswords}"
    regexp="^(manager\:.*)" select="\1" casesensitive="true" />

Kobi suggested adding -> flags=”m” <- which sounded good but ant reported that the flags option is not supported by propertyregex.

The final, successful, approach required inserting “(?m)” at the beginning of the regexp: That was the essential change.

<propertyregex property="manager.backup" input="${oldPasswords}"
    regexp="(?m)^manager:.*$" select="\0" casesensitive="true" />

The regexp with propertyregex appears to follow the rules in this documentation of regular expressions in Java (search for “multiline” for example): http://docs.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html

Check the above document if you have similar questions about how to make propertyregex and regexp do what you want them to do!

THANKS! Solved.

Alan Carwile

Answer 1

I think the m(ultiline) flag is what you want to use and will give the start-of-line anchor the right behavior. It’s possible to change flags within the regular expression with the syntax (?<flagstoturnon>-<flagstoturnoff>). So in your case, adding (?m) to the start of the regular expression (before the caret) should work.