I’m struggling to create a PCRE regex that finds a specific pattern at the start of lines, but only when the same pattern appeared in the previous line. The first occurrence should be ignored.
yeah, regex can be so confusing! i’d suggest checking out some online regex testers or forums for more support. sometimes just a little tweak can make a big difference!
Regex is great but contextual matching like this gets messy fast. I’ve dealt with similar parsing challenges and honestly, making regex handle state between lines usually becomes a maintenance nightmare.
I automate this now with a simple workflow that reads files line by line and tracks previous line state. Takes 5 minutes to set up and handles way more complex logic down the road.
You can build this in Latenode super easily. Create a workflow that:
Reads your text file
Splits it into lines
Loops through each line with a variable tracking the previous line
Outputs matches when current line has your pattern AND previous line had it too
I use this for log parsing all the time. Way more reliable than wrangling regex engines across different environments. Plus you can add extra conditions later without rewriting some crazy regex pattern.
The visual workflow makes it simple to modify and other people can actually understand what it does.
This is a classic regex problem that needs lookahead assertions. You want to match lines where the current line starts with your pattern AND the previous line also had that same pattern.
Here’s the regex:
(?<=\nLabel X:.*\n)Label X:
Breaking it down:
(?<=\nLabel X:.*\n) is a positive lookbehind that checks if the previous line had “Label X:”
Label X: matches your target pattern
I’ve hit similar patterns parsing log files at work. The trick is using lookbehind assertions to check what came before without including it in your match.
If you’re working with multiline strings, use the multiline flag (m) so ^ and $ work correctly with line boundaries.
For more on how regex lookarounds work, this video breaks it down well:
Test this on regex101.com with your sample data and you’ll see it captures lines 2, 3, 7, and 10 like you wanted.
I ran into the same thing parsing config files with repeated sections. The lookbehind approach should work, but you might hit issues with variable-length lookbehinds depending on your regex engine.
What worked for me was a two-step approach: capture everything with line numbers first, then filter it programmatically. If you’re stuck with pure regex though, try this:
(?m)^(Label X:)(?=.*\n\1)
This uses positive lookahead to check if the same pattern shows up on the next line. The (?m) turns on multiline mode so ^ actually matches line starts.
Testing across different regex flavors was huge - PCRE handles some stuff differently than other engines. Honestly though, regex alone isn’t always the most maintainable fix for contextual matching like this. Sometimes a simple script that tracks the previous line state is way cleaner and easier to read.
Had the same problem parsing log files with repeated error patterns. Lookbehind gets weird with multiline content in some PCRE implementations. Here’s what worked for me - use capturing groups that handle line boundaries better:
(^Label X:.*\n)(^Label X:.*)
This grabs both the previous line and target line as separate groups. Just pull group 2 for your matches. Instead of relying on lookbehind across line boundaries, you’re explicitly handling the newline character.
I tested this extensively on server logs with similar repeating patterns. It’s way more predictable across PCRE versions and avoids those variable-length lookbehind headaches. Just remember to enable multiline mode so the caret anchors work at line starts.
