I am trying to write a regex which should detect "Is the entire string a placeholder".

An example of a valid placeholder here is ${var} An example of an invalid palceholder here is ${var}-sometext as the placeholder is just a part of the text

The regex I have currently is ^\$\{(. )\}$

This works for normal cases. for example

1	${var}	Regex Matches	Expected ✅
2	${var} txt	Regex Does Not Match	Expected ✅

even works for nested placeholders

3	${var-${nestedVar}}	Regex Matches	Expected ✅

Where this fails is if the strings begins and ends with a placeholder for eg

4	${var1}-txt-${var2}	Regex Matches	NOT Expected ❌

Basically even though the entire string is not a placeholder, the regex treats it as one as it begins with ${ and ends with }

I can try solving it by replacing . with something like [^$] to exclude dollar, but that will break the nested use case in example 3.

How do I solve this?

EDIT

Adding some code for context

public static final Pattern PATTERN = Pattern.compile("^\\$\\{(. )\\}$");
Matcher matcher = PATTERN.matcher(placeholder);
boolean isMatch = matcher.find();

CodePudding user response：

From your example, I think you need to avoid greedy quantifier:

\$\{(. ?)\}

Notice the ? after which are reluctant quantifier: https://docs.oracle.com/javase/tutorial/essential/regex/quant.html

That should match ${var1}-txt-${var2}

Now, if you use ^ and $ as well, this will fail.

Note that you could also use StringSubstitutor from commons-text to perform a similar job (it will handle the parsing and you may use a Lookup that capture the variable).

CodePudding user response：

Please, never say never guys:

^\$\{[^\}] \}[\}] $

it just matchs everything inside ${, and just accepts } closes at end, looks perfect fits!

${var} -> Matches

${var-${anotherVar}} -> Matches

${var} txt -> Not Matches

${var}-txt-${var2} -> Not Matches

even infinity nested will matches:

${var-${nestedVar-${deep1-${deep2}}}} -> Matches