Powerful Regular Expressions Combined with Lisp in Emacs

Regular expressions are a powerful text transformation tool. Any UNIX geek will tell you that. It’s so deeply ingrained into our culture, that we even make jokes about it. Another thing that we also love is having a powerful extension language at hand, and Lisp is one of the most powerful extension languages around (and of course, we make jokes about that too).

Emacs, one of the most famous Lisp applications today, has for a while now the ability to combine both of these, to reach entirely new levels of usefulness. Combining regular expressions and Lisp can do really magical things.

An example that I recently used a few times is parsing & de-humanizing numbers in dstat output. The output of dstat includes numbers that are printed with a suffix, like ‘B’ for bytes, ‘k’ for kilobytes and ‘M’ for megabytes, e.g.:

----system---- ----total-cpu-usage---- --net/eth0- -dsk/total- sda-
     time     |usr sys idl wai hiq siq| recv  send| read  writ|util
16-05 08:36:15|  2   3  96   0   0   0|  66B  178B|   0     0 |   0
16-05 08:36:16| 42  14  37   0   0   7|  92M 1268k|   0     0 |   0
16-05 08:36:17| 45  11  36   0   0   7|  76M 1135k|   0     0 |   0
16-05 08:36:18| 27  55   8   0   0  11|  67M  754k|   0    99M|79.6
16-05 08:36:19| 29  41  16   5   0  10| 113M 2079k|4096B   63M|59.6
16-05 08:36:20| 28  48  12   4   0   8|  58M  397k|   0    95M|76.0
16-05 08:36:21| 38  37  14   1   0  10| 114M 2620k|4096B   52M|23.2
16-05 08:36:22| 37  54   0   1   0   8|  76M 1506k|8192B   76M|33.6

So if you want to graph one of the columns, it’s useful to convert all the numbers in the same unit. Bytes would be nice in this case.

Separating all columns with ‘|’ characters is a good start, so you can use e.g. a CSV-capable graphing tool, or even simple awk scripts to extract a specific column. ‘C-x r t’ can do that in Emacs, and you end up with something like this:

|     time     |cpu|cpu|cpu|cpu|cpu|cpu|eth0 |eth0 | disk| disk|sda-|
|     time     |usr|sys|idl|wai|hiq|siq| recv| send| read| writ|util|
|16-05 08:36:15|  2|  3| 96|  0|  0|  0|  66B| 178B|   0 |   0 |   0|
|16-05 08:36:16| 42| 14| 37|  0|  0|  7|  92M|1268k|   0 |   0 |   0|
|16-05 08:36:17| 45| 11| 36|  0|  0|  7|  76M|1135k|   0 |   0 |   0|
|16-05 08:36:18| 27| 55|  8|  0|  0| 11|  67M| 754k|   0 |  99M|79.6|
|16-05 08:36:19| 29| 41| 16|  5|  0| 10| 113M|2079k|4096B|  63M|59.6|
|16-05 08:36:20| 28| 48| 12|  4|  0|  8|  58M| 397k|   0 |  95M|76.0|
|16-05 08:36:21| 38| 37| 14|  1|  0| 10| 114M|2620k|4096B|  52M|23.2|
|16-05 08:36:22| 37| 54|  0|  1|  0|  8|  76M|1506k|8192B|  76M|33.6|

The leading and trailing ‘|’ characters are there so we can later use orgtbl-mode, an awesome table editing and realignment tool of Emacs. Now to the really magical step: regular expressions and lisp working together.

What we would like to do is convert text like “408B” to just “408″, text like “1268k” to the value of (1268 * 1024), and finally text like “67M” to the value of (67 * 1024 * 1024). The first part is easy:

M-x replace-regexp RET \([0-9]+\)B RET \1 RET

This should just strip the “B” suffix from byte values.

For the kilobyte and megabyte values what we would like is to be able to evaluate an arithmetic expression that involves \1. Something like “replace \1 with the value of (expression \1)“. This is possible in Emacs by prefixing the substitution pattern with \,. This instructs Emacs to evaluate the rest of the substitution pattern as a Lisp expression, and use its string representation as the “real” substitution text.

So if we match all numeric values that are suffixed by ‘k’, we can use (string-to-number \1) to convert the matching digits to an integer, multiply by 1024 and insert the resulting value by using the following substitution pattern:

\,(* 1024 (string-to-number \1))

The full Emacs command would then become:

M-x replace-regexp RET \([0-9]+\)k RET \,(* 1024 (string-to-number \1)) RET

This, and the byte suffix removal, yield now the following text in our Emacs buffer:

|     time     |cpu|cpu|cpu|cpu|cpu|cpu|eth0 |eth0 | disk| disk|sda-|
|     time     |usr|sys|idl|wai|hiq|siq| recv| send| read| writ|util|
|16-05 08:36:15|  2|  3| 96|  0|  0|  0|  66| 178|   0 |   0 |   0|
|16-05 08:36:16| 42| 14| 37|  0|  0|  7|  92M|1298432|   0 |   0 |   0|
|16-05 08:36:17| 45| 11| 36|  0|  0|  7|  76M|1162240|   0 |   0 |   0|
|16-05 08:36:18| 27| 55|  8|  0|  0| 11|  67M| 772096|   0 |  99M|79.6|
|16-05 08:36:19| 29| 41| 16|  5|  0| 10| 113M|2128896|4096|  63M|59.6|
|16-05 08:36:20| 28| 48| 12|  4|  0|  8|  58M| 406528|   0 |  95M|76.0|
|16-05 08:36:21| 38| 37| 14|  1|  0| 10| 114M|2682880|4096|  52M|23.2|
|16-05 08:36:22| 37| 54|  0|  1|  0|  8|  76M|1542144|8192|  76M|33.6|

Note: Some of the columns are indeed not aligned very well. We’ll fix that later. On to the megabyte conversion:

M-x replace-regexp RET \([0-9]+\)M RET \,(* 1024 1024 (string-to-number \1)) RET

Which produces a version that has no suffixes at all:

|     time     |cpu|cpu|cpu|cpu|cpu|cpu|eth0 |eth0 | disk| disk|sda-|
|     time     |usr|sys|idl|wai|hiq|siq| recv| send| read| writ|util|
|16-05 08:36:15|  2|  3| 96|  0|  0|  0|  66| 178|   0 |   0 |   0|
|16-05 08:36:16| 42| 14| 37|  0|  0|  7|  96468992|1298432|   0 |   0 |   0|
|16-05 08:36:17| 45| 11| 36|  0|  0|  7|  79691776|1162240|   0 |   0 |   0|
|16-05 08:36:18| 27| 55|  8|  0|  0| 11|  70254592| 772096|   0 |  103809024|79.6|
|16-05 08:36:19| 29| 41| 16|  5|  0| 10| 118489088|2128896|4096|  66060288|59.6|
|16-05 08:36:20| 28| 48| 12|  4|  0|  8|  60817408| 406528|   0 |  99614720|76.0|
|16-05 08:36:21| 38| 37| 14|  1|  0| 10| 119537664|2682880|4096|  54525952|23.2|
|16-05 08:36:22| 37| 54|  0|  1|  0|  8|  79691776|1542144|8192|  79691776|33.6|

Finally, to align everything in neat, pipe-separated columns, we enable M-x orgtbl-mode, and type “C-c C-c” with the pointer somewhere inside the transformed dstat output. The buffer now becomes something usable for pretty-much any graphing tool out there:

| time           | cpu | cpu | cpu | cpu | cpu | cpu |      eth0 |    eth0 |  disk |      disk | sda- |
| time           | usr | sys | idl | wai | hiq | siq |      recv |    send |  read |      writ | util |
| 16-05 08:36:15 |   2 |   3 |  96 |   0 |   0 |   0 |        66 |     178 |     0 |         0 |    0 |
| 16-05 08:36:16 |  42 |  14 |  37 |   0 |   0 |   7 |  96468992 | 1298432 |     0 |         0 |    0 |
| 16-05 08:36:17 |  45 |  11 |  36 |   0 |   0 |   7 |  79691776 | 1162240 |     0 |         0 |    0 |
| 16-05 08:36:18 |  27 |  55 |   8 |   0 |   0 |  11 |  70254592 |  772096 |     0 | 103809024 | 79.6 |
| 16-05 08:36:19 |  29 |  41 |  16 |   5 |   0 |  10 | 118489088 | 2128896 |  4096 |  66060288 | 59.6 |
| 16-05 08:36:20 |  28 |  48 |  12 |   4 |   0 |   8 |  60817408 |  406528 |     0 |  99614720 | 76.0 |
| 16-05 08:36:21 |  38 |  37 |  14 |   1 |   0 |  10 | 119537664 | 2682880 |  4096 |  54525952 | 23.2 |
| 16-05 08:36:22 |  37 |  54 |   0 |   1 |   0 |   8 |  79691776 | 1542144 |  8192 |  79691776 | 33.6 |

The trick of combining arbitrary Lisp expressions with regexp substitution patterns like \1, \2\9 is something I have found immensely useful in Emacs. Now that you know how it works, I hope you can find even more amusing use-cases for it.

Update: The Emacs manual has a few more useful examples of \, in action, as pointed out by tunixman on Twitter.

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