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Expression Functions File

A functions file allows you to specify additional expression helpers that can be loaded and used in rare.


For example, if you frequently need to double a number you could create a function:

double {multi {0} 2}

And use it in rare with argument --funcs:

rare --funcs custom.funcs filter -m '(\d+)' -e '{double {1}}' access.log

Or via environment variable RARE_FUNC_FILES:

export RARE_FUNC_FILES=/path/to/custom.funcs
rare filter -m '(\d+)' -e '{double {1}}' access.log

You can load multiple files by providing --funcs argument multiple times, or providing a comma-separated list to $RARE_FUNC_FILES


To validate that your function(s) are showing up as expected, you can test them with rare expr ... or list them by running rare expr --listfuncs.


A functions file is key-value pairs of name to expression. Lines starting with #, or any characters after #, are considered comments.

Expressions can be multi-line by ending the previous line with a \.

Integer keys (eg. {0}) refer to arguments passed into the function. Named keys (eg. {line}) will pass-through to the match context.

# Allows comments that start with '#'
name-of-func {sumi {0} {1}} # comments can also go here

# Multi-line ends with '\'
classifylen {switch \
    # short string
    {lt {len {0}} 5} short \
    # long string
    {gt {len {0}} 15} long \
    medium \ # else, medium