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Document a thousands-separator recipe for the DSL (#382)

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Go's fmt package doesn't support the printf apostrophe flag (%'d), and
Go's RE2 regex engine doesn't support the lookaheads used by the
well-known sed/perl grouping recipe. Add a docs section showing a small
user-defined DSL function which inserts thousands separators via
repeated sub() application, handling negatives and decimal parts, and
passing non-numeric values through unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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John Kerl 2026-07-06 15:38:49 -04:00
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39
docs/src/reference-main-number-formatting.md
View file

@ -84,3 +84,42 @@ x=0xffff,y=0xff,z=16711425
<pre class="pre-non-highlight-in-pair">
x=0xffff,y=0xff,z=0xfeff01
</pre>
## Thousands separators
Some tools accept a `printf`-style apostrophe flag, e.g. `%'d`, to insert
locale-dependent thousands separators, formatting `12345` as `12,345`. Since
Miller's number formatting uses Go's [fmt](https://pkg.go.dev/fmt) package,
which doesn't support that flag, formats like `--ofmt "%'d"`,
`fmtnum($x, "%'d")`, and `format-values -i "%'d"` won't work. Likewise, the
well-known `sed`/`perl` recipe for inserting separators relies on regular-expression
lookaheads such as `(?=...)`, which [Go's regular-expression engine](reference-main-regular-expressions.md)
doesn't support.
You can, however, insert thousands separators with a small user-defined
[DSL function](reference-dsl-user-defined-functions.md) which applies
[`sub`](reference-dsl-builtin-functions.md#sub) until there's nothing left to
do. Digits are grouped from the left, so the decimal part (if any) is
untouched, and non-numeric values pass through unchanged:
<pre class="pre-highlight-in-pair">
<b>echo 'x=12345,y=1234567.8901,z=-987654321,s=hello' | mlr --opprint put '</b>
<b> func commify(n) {</b>
<b> s = string(n);</b>
<b> while (s =~ "^(-?[0-9]+)([0-9]{3})") {</b>
<b> s = sub(s, "^(-?[0-9]+)([0-9]{3})", "\1,\2");</b>
<b> }</b>
<b> return s;</b>
<b> }</b>
<b> for (k, v in $*) {</b>
<b> $[k] = commify(v);</b>
<b> }</b>
<b>'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
x y z s
12,345 1,234,567.8901 -987,654,321 hello
</pre>
To use a separator other than comma -- say, a space or a period -- simply
change the `,` in the `"\1,\2"` replacement string.

35
docs/src/reference-main-number-formatting.md.in
View file

@ -50,3 +50,38 @@ GENMD-EOF
GENMD-RUN-COMMAND
echo 'x=0xffff,y=0xff' | mlr put '$z=hexfmt($x*$y)'
GENMD-EOF
## Thousands separators
Some tools accept a `printf`-style apostrophe flag, e.g. `%'d`, to insert
locale-dependent thousands separators, formatting `12345` as `12,345`. Since
Miller's number formatting uses Go's [fmt](https://pkg.go.dev/fmt) package,
which doesn't support that flag, formats like `--ofmt "%'d"`,
`fmtnum($x, "%'d")`, and `format-values -i "%'d"` won't work. Likewise, the
well-known `sed`/`perl` recipe for inserting separators relies on regular-expression
lookaheads such as `(?=...)`, which [Go's regular-expression engine](reference-main-regular-expressions.md)
doesn't support.
You can, however, insert thousands separators with a small user-defined
[DSL function](reference-dsl-user-defined-functions.md) which applies
[`sub`](reference-dsl-builtin-functions.md#sub) until there's nothing left to
do. Digits are grouped from the left, so the decimal part (if any) is
untouched, and non-numeric values pass through unchanged:
GENMD-RUN-COMMAND
echo 'x=12345,y=1234567.8901,z=-987654321,s=hello' | mlr --opprint put '
func commify(n) {
s = string(n);
while (s =~ "^(-?[0-9]+)([0-9]{3})") {
s = sub(s, "^(-?[0-9]+)([0-9]{3})", "\1,\2");
}
return s;
}
for (k, v in $*) {
$[k] = commify(v);
}
'
GENMD-EOF
To use a separator other than comma -- say, a space or a period -- simply
change the `,` in the `"\1,\2"` replacement string.