Yesterday, I encountered an analysis issue that appeared to be resolvable with a simple pattern replacement technique. 

This turned out to be a bit more complex than estimated, as the pattern spanned multiple lines and I struggled to get the regex constructed to match this use case.

I finally figured it out, and although the overall problem remains unresolved (it has many layers), this particular part works well.  My error was in how I viewed use of  /s and /m as mutually exclusive; they aren’t of course, and I have made that mistake one other time over the years.   I also hadn’t set the input record separator ($/) to paragraph read mode.   I’m posting details here to document my specific usage.

Assumptions:

• A literal pattern exists that is present in multiple places in a file
• The literal pattern spans multiple lines in the file
• Literal pattern is
  • Node-path: branches/%%BRANCHES%%
  • Node-kind: dir
  • Node-action: add
• Perl is being used to read the file and current line of file is $line
• Objective is to delete all occurrences of this pattern from the file one $line at a time

Here’s how I did that:

[read the file & crawl thru the lines]

$/ = ”;

$line =~ s/^Node-path:\sbranches\/%%BRANCHES%%.*?^Node-kind:\sdir.*?^Node-action:\sadd.*?//sm;

Tags: perl, regex, tech-tips

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