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;doc: clarify when regexps do and do not support backreferences

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Simon Michael 2020-05-09 15:35:53 -07:00
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hledger/hledger.m4.md
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@ -827,14 +827,23 @@ hledger uses [regular expressions](http://www.regular-expressions.info) in a num
hledger's regular expressions come from the
[regex-tdfa](http://hackage.haskell.org/package/regex-tdfa/docs/Text-Regex-TDFA.html)
library. In general they:
library.
If they're not doing what you expect, it's important to know exactly what they support:
- are case insensitive
- are infix matching (do not need to match the entire thing being matched)
- are [POSIX extended regular expressions](http://www.regular-expressions.info/posix.html#ere)
- also support [GNU word boundaries](http://www.regular-expressions.info/wordboundaries.html) (\\<, \\>, \\b, \\B)
- and parenthesised [capturing groups](http://www.regular-expressions.info/refcapture.html) and numeric backreferences in replacement strings
- do not support [mode modifiers](http://www.regular-expressions.info/modifiers.html) like (?s)
#. they are case insensitive
#. they are infix matching (they do not need to match the entire thing being matched)
#. they are [POSIX extended regular expressions][]
#. they also support [word boundaries][] (`\<`, `\>`, `\b`, `\B`)
#. they do not support [mode modifiers][] (like `(?s)`)
#. when searching, they do not support [capturing groups][] and [backreferences][] (`\1`).
But when used for text replacement (eg in [account aliases](journal.html#regex-aliases)),
they do support [capturing groups][] in the search string and [backreferences][] in the replacement string.
[POSIX extended regular expressions]: http://www.regular-expressions.info/posix.html#ere
[backreferences]: https://www.regular-expressions.info/backref.html
[capturing groups]: http://www.regular-expressions.info/refcapture.html
[mode modifiers]: http://www.regular-expressions.info/modifiers.html
[word boundaries]: http://www.regular-expressions.info/wordboundaries.html
Some things to note: