journal: tests/docs for aliases corrupting account names (#1788)

hledger/Hledger/Cli/Commands/Print.md

@@ -58,6 +58,7 @@ There are some situations where print's output can become unparseable:
 
 - [Valuation](#valuation) affects posting amounts but not [balance assertion](#balance-assertions) or [balance assignment](#balance-assignments) amounts, potentially causing those to [fail](https://github.com/simonmichael/hledger/issues/1429).
 - [Auto postings](#auto-postings) can generate postings with [too many missing amounts](https://github.com/simonmichael/hledger/issues/1276).
+- [Account aliases can generate invalid account names](#account-aliases-can-generate-invalid-account-names).
 
 Normally, the journal entry's explicit or implicit amount style is preserved.
 For example, when an amount is omitted in the journal, it will not appear in the output.

hledger/hledger.m4.md

@@ -3153,12 +3153,17 @@ This can be useful for:
 
 - expanding shorthand account names to their full form, allowing easier data entry and a less verbose journal
 - adapting old journals to your current chart of accounts
-- experimenting with new account organisations, like a new hierarchy or combining two accounts into one
+- experimenting with new account organisations, like a new hierarchy
+- combining two accounts into one, eg to see their sum or difference on one line
 - customising reports
 
 Account aliases also rewrite account names in [account directives](#declaring-accounts).
 They do not affect account names being entered via hledger add or hledger-web.
 
+Account aliases are very powerful.
+They are generally easy to use correctly, but you can also generate 
+invalid account names with them; more on this below.
+
 See also [Rewrite account names](rewrite-account-names.html).
 
 ### Basic aliases
@@ -3276,6 +3281,39 @@ with this directive:
 end aliases
 ```
 
+### Aliases can generate invalid account names
+
+Be aware that account aliases can produce malformed account names,
+which could cause confusing reports or and invalid [`print`](#print) output.
+Two examples: you can erase an account name:
+
+```journal
+2021-01-01
+  a:aa     1
+  b
+```
+```shell
+$ hledger -f- print --alias '/a:.*/='
+2021-01-01
+                    1
+    b
+```
+
+or insert an illegal double space, causing part of the account name
+to be treated as part of the amount if reparsed:
+
+```journal
+2021-01-01
+  old    1
+  other
+```
+```shell
+$ hledger -f- --alias old="new  USD" print | hledger -f- print
+2021-01-01
+    new             USD 1
+    other
+```
+
 ## Default parent account
 
 You can specify a parent account which will be prepended to all accounts

hledger/test/account-aliases.test

@@ -225,3 +225,30 @@ $ hledger -f- print
 
 >=0
 
+# Aliases can produce malformed account names and invalid print output (#1788).
+# 16. For example, you can erase an account name:
+<
+alias /a:.*/=
+2021-01-01
+  a:aa     1
+  b
+$ hledger -f- print
+2021-01-01
+                    1
+    b
+
+>=0
+
+# 17. Another example: you can insert an illegal double space, 
+# with the second part considered part of the amount if reparsed:
+<
+2021-01-01
+  old    1
+  other
+
+$ hledger -f- --alias old="new  USD" print | hledger -f- print
+2021-01-01
+    new             USD 1
+    other
+
+>=0