Is it possible to use Unicode character escaping (e.g. \u2009) in YAML multiline strings?
this_escape_works: "foo\u2009bar"
this_escape_doesnt: >
foo\u2009bar
As per the YAML1.2 spec double quoted style for scalars is:
the only style capable of expressing arbitrary strings, by using “\” escape sequences. This comes at the cost of having to escape the “\” and “"” characters.
So folded scalars do not support escaping and you have to do
this_escape_works: "foo\u2009bar"
this_escape_doesnt: "foo\u2009bar\n"
Please note that contrary to #nj_'s claim the folding indicator > (it is not an operator) doesn't do "newlines become spaces" in general and certainly not for the final line break
Related
Both of these commands doesn't create multiple line text:
Set-Content .\test.md 'Hello`r`nWorld'
Set-Content .\test.md 'Hello\r\nWorld'
Only this can
Set-Content .\test.md #("Hello`nWorld")
Do you know why is that?
Escape sequences such as `r`n only work inside "...", i.e, expandable (interpolating) strings.
By contrast, '...' strings are verbatim strings that do not interpret their contents - even ` instances are used as verbatim (literally).
Only ` (the so-called backtick) serves as the escape character in PowerShell, not \.
That is, in both "..." and '...' strings a \ is a literal.
(However, \ is the escape character in the context of regexes (regular expressions), but it is then the .NET regex engine that interprets them, not PowerShell; e.g.,
"`r" -match '\r' is $true: the (interpolated) literal CR char. matched its escaped regex representation).
As for what you tried:
It is the fact that "Hello`nWorld" in your last command is a "..." string that made it work.
By contrast, enclosing the string in #(...), the array-subexpression operator, is incidental to the solution. (Set-Content's (positionally implied) -Value parameter is array-valued anyway (System.Object[]), so even a single string getting passed is coerced to an array).
Finally, note that Set-Content by default adds a trailing, platform-native newline to the output file; use -NoNewLine to suppress that, but note that doing so also places no newline between the (string representations of) multiple input objects, if applicable (in your case there's only one).
Therefore (note the -NoNewLine and the trailing `n):
Set-Content -NoNewLine .\test.md "Hello`nWorld`n"
Optional reading: design rationale for PowerShell's behavior:
Why doesn't PowerShell use the backslash (\) as the escape character, like other languages?
Because PowerShell must (also) function on Windows (it started out as Windows-only), use of \ as the escape character - as known from Unix (POSIX-compatible) shells such as Bash - is not an option, given that \ is used as the path separator on Windows.
If \ were the escape character, you'd have to use Get-ChildItem C:\\Windows\\System32 instead of Get-ChildItem C:\Windows\System32, which is obviously impractical in a shell, where dealing with file-system paths is very common.
Thus, a different character had to be chosen, which turned out to be `, the so-called backtick: At least on US keyboards, it is easy to type (just like \), and it has the benefit of occurring rarely (as itself) in real-world strings, so that the need to escape it rarely arises.
Note that the much older legacy shell on Windows, cmd.exe, too had to pick a different character: it chose ^, the so-called caret.
Why doesn't it use single quote and double quote interchangeably, like other languages?
Different languages made different design choices, but in making "..." strings interpolating, but '...' strings not, PowerShell did follow existing languages here, namely that of POSIX-compatible shells such as Bash.
As an improvement on the latter PowerShell also supports embedding verbatim ' inside '...', escaped as '' (e.g., '6'' tall')
Given PowerShell's commitment to backward compatibility, this behavior won't change, especially given how fundamental it is to the language.
Conceptually speaking, you could argue that the aspect of what quoting character a string uses should be separate from whether it is interpolating, so that you'd be free to situationally choose one or the other quoting style for syntactic convenience, while separately controlling whether interpolation should occur.
Thus, hypothetically, PowerShell could have used a separate sigil to make a string interpolating, say $"..." and $'...' (similar to what C# now offers, though it notably only has one string-quoting style).
(As an aside: Bash and Ksh do have this syntax form, but it serves a different purpose (localization of strings) and is rarely used in pratice).
In practice, however, once you know how "..." and '...' work in PowerShell, it isn't hard to make them work as intended.
See this answer for a juxtaposition of PowerShell, cmd.exe, and POSIX-compatible shells with respect to fundamental features.
The following match returns false. How can I change the regular expression to correct it?
"hello$world" -match '^hello$(wo|ab).*$'
"hello$abcde" -match '^hello$(wo|ab).*$'
'hello$world' -match '^hello\$(wo|ab).*$'
'hello$abcde' -match '^hello\$(wo|ab).*$'
You need to quote the left hand side with single quotes so $world isn't treated like variable interpolation. You need to escape the $ in the right hand side so it isn't treated as end of line.
From About Quoting Rules:
When you enclose a string in double quotation marks (a double-quoted string), variable names that are preceded by a dollar sign ($) are replaced with the variable's value before the string is passed to the command for processing.
...
When you enclose a string in single-quotation marks (a single-quoted string), the string is passed to the command exactly as you type it. No substitution is performed.
From About Regular Expressions:
The two commonly used anchors are ^ and $. The carat ^ matches the start of a string, and $, which matches the end of a string. This allows you to match your text at a specific position while also discarding unwanted characters.
...
Escaping characters
The backslash \ is used to escape characters so they are not parsed by the regular expression engine.
The following characters are reserved: []().\^$|?*+{}.
You will need to escape these characters in your patterns to match them in your input strings.
Under what circumstances must one use quotes in a YAML file, specifically when using docker-compose.
For instance,
service:
image: "my-registry/repo:tag1"
environment:
ENV1: abc
ENV2: "abc"
ENV3: "a b c"
If spaces are required, for example, must one use quotes around the environment variable, as depicted in ENV3?
After some googling I've found a blog post
that touches this problem as I understood it.
I'll cite the most important part here:
plain scalars:
- a string
- a string with a \ backslash that doesn't need to be escaped
- can also use " quotes ' and $ a % lot /&?+ of other {} [] stuff
single quoted:
- '& starts with a special character, needs quotes'
- 'this \ backslash also does not need to be escaped'
- 'just like the " double quote'
- 'to express one single quote, use '' two of them'
double quoted:
- "here we can use predefined escape sequences like \t \n \b"
- "or generic escape sequences \x0b \u0041 \U00000041"
- "the double quote \" needs to be escaped"
- "just like the \\ backslash"
- "the single quote ' and other characters must not be escaped"
literal block scalar: |
a multiline text
line 2
line 3
folded block scalar: >
a long line split into
several short
lines for readability
Also I have not seen such docker-compose syntax to set env variables. Documentation suggests using simple values like
environment:
- ENV1=abc
- "ENV2=abc"
Where quotes " or ' are optional in this particular example according to what I've said earlier.
To see how to include spaces in env variables you can check out this so answer
Whether or not you need quotes, depends on the parser. Docker-compose AFAIK is still relying on the PyYAML module and that implements most of YAML 1.1 and has a few quirks of its own.
In general you only need to quote what could otherwise be misinterpreted or clash with some YAML construct that is not a scalar string. You also need (double) quotes for things that cannot be represented in plain scalars, single quoted scalars or block style literal or folded scalars.
Misinterpretation
You need to quote strings that look like some of the other data structures:
booleans: "True", "False", but PyYAML also assumes alternatives words like "Yes", "No", "On", "Off" represent boolean values ( and the all lowercase, all uppercase versions should be considered as well). Please note that the YAML 1.2 standard removed references to these alternatives.
integers: this includes string consisting of numbers only. But also hex (0x123) and octal number (0123). The octals in YAML 1.2 are written as 0o123, but PyYAML doesn't support this, however it is best to quote both.
A special integer that PyYAML still supports but again not in the YAML 1.2 specification are sexagesimals: base 60 number separated by colon (:), time indications, but also MAC addresses can be interpreted as such if the values between/after the colons are in the range 00-59
floats: strings like 1E3 (with optional sign ans mantissa) should be quoted. Of course 3.14 needs to be quoted as well if it is a string. And sexagesimal floats (with a mantissa after the number after the final colon) should be quoted as well.
timestamps: 2001-12-15T02:59:43.1Z but also iso-8601 like strings should be quoted to prevent them from being interpreted as timestamps
The null value is written as the empty string, as ~ or Null (in all casing types), so any strings matching those need to be quoted.
Quoting in the above can be done with either single or double quotes, or block style literal or folded scalars can be used. Please note that for the block-style you should use |- resp. >- in order not to introduce a trailing newline that is not in the original string.
Clashes
YAML assigns special meaning to certain characters or character combinations. Some of these only have special meaning at the beginning of a string, others only within a string.
characters fromt the set !&*?{[ normally indicate special YAML constructs. Some of these might be disambiguated depending on the following character, but I would not rely on that.
whitespace followed by # indicates an end of line comment
wherever a key is possible (and within block mode that is in many places) the combination of colon + space (:) indicates a value will be following. If that combination is part of your scalar string, you have to quote.
As with the misinterpretation you can use single or double quoting or block-style literal or folding scalars. There can be no end-of-line comments beyond the first line of a block-style scalar.
PyYAML can additionally get confused by any colon + space within a plain scalar (even when this is in a value) so always quote those.
Representing special characters
You can insert special characters or unicode code-points in a YAML file, but if you want these to be clearly visible in all cases, you might want to use escape sequences. In that case you have to use double quotes, this is the only mode that
allows backslash escapes. And e.g. \u2029. A full list of such escapes can be taken from the standard, but note that PyYAML doesn't implement e.g \/ (or at least did not when I forked that library).
One trick to find out what to quote or not is to use the library used to dump the strings that you have. My ruamel.yaml and PyYAML used by docker-compose, when potentially dumping a plain scalar, both try to read back (yes, by parsing the result) the plain scalar representation of a string and if that results in something different than a string, it is clear quotes need to be applied. You can do so too: when in doubt write a small program dumping the list of strings that you have using PyYAML's safe_dump() and apply quotes anywhere that PyYAML does.
Question (because I can't work it out), should ""hello world"" be a valid field value in a CSV file according to the specification?
i.e should:
1,""hello world"",9.5
be a valid CSV record?
(If so, then the Perl CSV-XS parser I'm using is mildly broken, but if not, then $line =~ s/\342\200\234/""/g; is a really bad idea ;) )
The weird thing is is that this code has been running without issue for years, but we've only just hit a record that started with both a left double quote and contained no comma (the above is from a CSV pre-parser).
The canonical format definition of CSV is https://www.rfc-editor.org/rfc/rfc4180.txt. It says:
Each field may or may not be enclosed in double quotes (however
some programs, such as Microsoft Excel, do not use double quotes
at all). If fields are not enclosed with double quotes, then
double quotes may not appear inside the fields. For example:
"aaa","bbb","ccc" CRLF
zzz,yyy,xxx
Fields containing line breaks (CRLF), double quotes, and commas
should be enclosed in double-quotes. For example:
"aaa","b CRLF
bb","ccc" CRLF
zzz,yyy,xxx
If double-quotes are used to enclose fields, then a double-quote
appearing inside a field must be escaped by preceding it with
another double quote. For example:
"aaa","b""bb","ccc"
Last rule means your line should have been:
1,"""hello world""",9.5
But not all parsers/generators follow this standard perfectly, so you might need for interoperability reasons to relax some rules. It all depends on how much you control the CSV format writing and CSV format parsing parts.
That depends on the escape character you use. If your escape character is '"' (double quote) then your line should look like
1,"""hello world""",9.5
If your escape character is '\' (backslash) then your line should look like
1,"\"hello world\"",9.5
Check your parser/environment defaults or explicitly configure your parser with the escape character you need e.g. to use backslash do:
my $csv = Text::CSV_XS->new ({ quote_char => '"', escape_char => "\\" });
I have a replace statement in my code whereby Band's is being replaced by Cig's. However when I put single quote it took the first sentence... Example 'Band'
I tried to use double quote but it does not work. Do you know how to escape the single quote sign?
-replace 'Band's', 'Cig's'
See Escape characters, Delimiters and Quotes and Get-Help about_Quoting_Rules from the built-in help (as pointed out by as Nacimota).
To include a ' inside a single-quoted string, simply double it up as ''. (Single-quote literals don't support any of the other escape characters.)
> "Band's Toothpaste" -replace 'Band''s', 'Cig''s'
Or, simply switch to double-quotes. (Double-quote literals are required when wishing to use interpolation or escape characters.)
> "Band's Toothpaste" -replace "Band's", "Cig's"
(Don't forget that -replace uses a regular expression)
Escape a single quote with two single quotes:
"Band's Toothpaste" -replace 'Band''s', 'Cig''s'
Also, this is a duplicate of
Can I use a single quote in a Powershell 'string'?
For trivial cases, you can use embedded escape characters. For more complex cases, you can use here-strings.
$Find = [regex]::escape(#'
Band's
'#)
$Replace = #'
Cig's
'#
"Band's Toothpaste" -replace $Find,$Replace
Then put the literal text you want to search for and replace in the here-strings.
Normal quoting rules don't apply within the here-string #' - '# delimiters, so you can put whatever kind of quotes you want, wherever you want them without needing any escape characters.
The [regex]::excape() on $Find will take care of doing the backslash escapes on any regex reserved characters that might be in the search pattern.