How do you use a label as a string in a loki.process block? Attempting timestamp formatting

Grafana Alloy
1

Hi.
I have a log file with a custom format. The timestamps, per line, in the log file contain just time and no date info. My plan was to use a regex to extract the date information from the filename label, then reformat the timestamp entry. However I can not figure out how access the filename value in the loki.process block.

Here’s the current loki.process block from the config.alloy:

loki.process "timestamp_alignment" {

        forward_to = [loki.write.grafana_loki.receiver]

        stage.regex {
                source = "filename"
                expression = `log_(?P<file_year>\d{4})(?P<file_month>\d{2})(?P<file_day>\d{2})`
        }

        stage.regex {
                source = ""
                expression = `\[(?P<line_timestamp>\d{2}:\d{2}:\d{2}\.\d{3})`
        }

        stage.replace {
                expression = `\[(?P<line_timestamp>\d{2}:\d{2}:\d{2}\.\d{3})\]`
                replace    = "file_year" + "-" + "file_month" + "-" + "file_day" + "T" + "line_timestamp"
                //replace    = "2025-02-26" + "T" + "line_timestamp"
        }

                //RFC3339: 2006-01-02T15:04:05-07:00
                //2006-01-02T15:04:05.000000Z
        stage.timestamp {
                source = "time"
                location = "Africa/Abidjan"
                format = "RFC3339"
        }
}

Here’s the error I get. Note that line 56 is the start of the above process block:
alloy-1 | Error: /etc/alloy/config.alloy:56:1: Failed to build component: building component: invalid stage config empty source

What am I doing wrong? I can get the value of a label in a loki.relabel block, but not in a loki.process block.

Thank you in advance for your help.

2

where is your loki.source ? also please post some sample log lines