Regex expression to select pair of lines - regex

Question

I have a logfile. Its general format is
log text 1 <br/>
log text 2 <br/>
Error: xxxxxx <br/>
error description (1 line only) <br/>
log text 3 <br/>
log text 4 <br/>
.... <br/>
Error: xxxxxx <br/>
error description (1 line only) <br/>
log text 5 <br/>
.... <br/>
I would like to select and extract pair of 2 lines containing errors (each error has two lines. The first line always has Error: as keyword. Word Error does not occur anywhere else in logfile).
How do I do it using regex or any other way. I can use MacOS, Unix or Windows XP. MacOS preferred.

Answer 1

Using grep on mac or unix based OS
grep -i error -A2 inputfile

Answer 2

A regular expression to find those 2 lines is for example:
^.*?Error.*(?:\r?\n|\r).*$
^ ... start each search at beginning of a line.
.*? ... match any character except carriage return and line-feed zero or more times non greedy. Non greedy means as less characters as possible. In other words stop on first occurrence of Error and not on last occurrence.
Error ... this word must be found in the first of 2 lines to match.
.* ... match any character except carriage return and line-feed zero or more times greedy. Greedy means now match as many characters as possible.
(?:\r?\n|\r) ... is a non-marking group matching either carriage return + line-feed (DOS/Windows text files), or only line-feed (UNIX text files), or only carriage return (old MAC text file).
.* ... match any character except carriage return and line-feed 0 or more times greedy.
$ ... anchor for end of line. Line termination is not included in matched string.
In other words this expression matches an entire line containing anywhere the word Error, the line terminator of this first line and everything on next line up to end of line, but not matching also the line terminator of this second line.
UltraEdit is a shareware text editor available for Windows, Linux and Mac.
Using this Perl regular expression in Search - Find dialog of UltraEdit with advanced find option List lines containing string enabled results in getting all found 2 line strings written to a window listing all found lines.
Opening the context menu of this window (right click on Windows) and clicking on Copy to Clipboard results in copying all found lines to system clipboard.
Pressing Ctrl+N to open a new file, Ctrl+V to paste the copied lines, and Ctrl+S to save the new file results finally in having a file with the wanted information.
Another method is using the UltraEdit script FindStringsToNewFile with the reduced regular expression search string Error.*(?:\r?\n|\r).*. This script writes all found strings starting with keyword Error and ending at end of next line directly to a new file.
One more note:
If a . (dot) matches also newline characters like carriage return and line-feed depends on a flag. In UltraEdit the flag is by default set that a dot does not match newline characters. With (?s) at beginning of a Perl regular expression search string the flag is changed and the dot would then match also newline characters for this search. With (?-s) at beginning of a search string the flag can be set for not matching newline characters by a dot if the internal default of the application is the opposite.