How to write this in different regex flavours

I have the following data:

a b c d FROM:<uniquepattern1>
e f g h TO:<uniquepattern2>
i j k l FROM:<uniquepattern1>
m n o p TO:<uniquepattern3>
q r s t FROM:<uniquepattern4>
u v w x TO:<uniquepattern5>

I would like a regex query that can find the contents of TO: when FROM:<uniquepattern1> is encountered, so the results would be uniquepattern2 and uniquepattern3.

I am hopeles开发者_如何学JAVAs with regex, I would appreciate any pointers on how to write this (lookahead parameters?) and any differences between regex on different platforms (eg the C# .NET Regex versus Grep vs Perl) that might be relevant here.

Thank you.

Try:

/FROM:<uniquepattern1>.*\r?\n.*?TO:<(.*?)>/

This works by first finding the FROM anchor and then use a dot wildcard. The dot operator does not match a newline so this will consume the rest of the line. A non-greedy dot wildcard match then consumes up to the next TO and captures what's between the angle brackets.

your requirement for file parsing is simple. there is no need to use regular expression. Open the file for reading, go through each line check for FROM:<uniquepattern1>, get the next line and print them out. Furthermore, your TO lines are only separated by ":". therefore you can use that as field delimiter.

eg with awk

$ awk -F":" '/FROM:<uniquepattern1>/{getline;print $2}' file
<uniquepattern2>
<uniquepattern3>

the same goes for other languages/tools