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;doc: import: document Match Groups

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Add a description of Match Groups to the manual; Section "Matchers".
Include two examples.

Clarify a description of regular expression features with respect
to match groups.

Expand the description of field assignments to cover match group
interpolation, cross-referencing to Section "Matchers" for the full
description.

Signed-off-by: Jonathan Dowland <jon@dow.land>
This commit is contained in:
Jonathan Dowland 2023-10-30 13:50:04 +00:00 committed by Simon Michael
parent aaf50c165c
commit d424966706
1 changed files with 29 additions and 5 deletions
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hledger/hledger.m4.md
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@ -399,9 +399,10 @@ If they're not doing what you expect, it's important to know exactly what they s
2. they are infix matching (they do not need to match the entire thing being matched)
3. they are [POSIX ERE] (extended regular expressions)
4. they also support [GNU word boundaries] (`\b`, `\B`, `\<`, `\>`)
5. they do not support [backreferences]; if you write `\1`, it will match the digit `1`.
Except when doing text replacement, eg in [account aliases](#regex-aliases),
where [backreferences] can be used in the replacement string to reference [capturing groups] in the search regexp.
5. [backreferences] are supported when doing text replacement in [account
aliases](#regex-aliases) or [CSV rules](#csv-rules), where [backreferences]
can be used in the replacement string to reference [capturing groups] in the
search regexp. Otherwise, if you write `\1`, it will match the digit `1`.
6. they do not support [mode modifiers] (`(?s)`), character classes (`\w`, `\d`), or anything else not mentioned above.
[POSIX ERE]: http://www.regular-expressions.info/posix.html#ere
@ -3006,8 +3007,9 @@ To assign a value to a hledger field, write the [field name](#field-names)
(any of the standard hledger field/pseudo-field names, defined below),
a space, followed by a text value on the same line.
This text value may interpolate CSV fields,
referenced by their 1-based position in the CSV record (`%N`),
or by the name they were given in the fields list (`%CSVFIELD`).
referenced either by their 1-based position in the CSV record (`%N`)
or by the name they were given in the fields list (`%CSVFIELD`),
and regular expression [match groups](#match-groups) (`\N`).
Some examples:
@ -3259,6 +3261,28 @@ When an if block has multiple matchers, they are combined as follows:
When a matcher is preceded by an exclamation mark (!), the matcher will be negated, ie it will exclude CSV records that match.
### Match groups
Matchers can define match groups: parenthesised portions of the regular expression
which are available for reference in field assignments. Groups are enclosed
in regular parentheses (`(` and `)`) and can be nested. Each group is available
in field assignments using the token `\N`, where N is an index into the match groups
for this conditional block (e.g. `\1`, `\2`, etc.).
Example: Warp credit card payment postings to the beginning of the billing period (Month
start), to match how they are presented in statements, using [posting dates](#posting-dates):
```rules
if %date (....-..)-..
comment2 date:\1-01
```
Another example: Read the expense account from the CSV field, but throw away a prefix:
```rules
if %account1 liabilities:family:(expenses:.*)
account1 \1
```
## `if` table