# stringref

https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.4.7-stringref

`stringref` statement helps to get string values to the VFR code:
```
EFI_IFR_STRING_REF1

Summary:
Push a string on the expression stack.

Prototype:

#define EFI_IFR_STRING_REF1_OP 0x4e

typedef struct _EFI_IFR_STRING_REF1 {
 EFI_IFR_OP_HEADER Header;
 EFI_STRING_ID StringId;
} EFI_IFR_STRING_REF1;

Members:
Header 		The byte sequence that defines the type of opcode as well as the length of the opcode being defined.
		Header.OpCode = EFI_IFR_STRING_REF1_OP.
StringId 	The string’s identifier, which must be unique within the package list.

Description:
Push the string specified by StringId on to the expression stack. If the string does not exist, then push an empty string.
```

For example this condition in a `suppressif` statement is TRUE only if a string element has a specific data in itself:
```
string
  name = StringQuestion,
  ...
endstring;

suppressif questionref(StringQuestion) == stringref(STRING_TOKEN(TEST_STRING));
  ...
endif;
```

Off course you need to define this new token that we use here for comparision:
```
#string TEST_STRING            #language en-US  "EDKII"
```

Another example of potential `stringref` usage:
```
string
  varid = FormData.StringValue,
  prompt = STRING_TOKEN(STRING_PROMPT),
  help = STRING_TOKEN(STRING_HELP),
  minsize = 5,
  maxsize = 10,
  warningif
    prompt = STRING_TOKEN(WARNING_IF_PROMPT),
    pushthis == stringref(STRING_TOKEN(TEST_STRING))
  endif;
endstring;
```

This code outputs a warning message if input string is equal to "EDKII".

# toupper/tolower

https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.6.8-toupper

https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.6.9-tolower

The `toupper` and `tolower` builtin functions are encoded in `EFI_IFR_TO_UPPER` (=0x21) and `EFI_IFR_TO_LOWER` (=0x20) opcodes and help to change case-register of the string data.

For example this code will output warning on "EDKII" input as well as on "Edkii", "EDkii", "EDKii", "eDKii" and so on:
```
string
  varid = FormData.StringValue,
  prompt = STRING_TOKEN(STRING_PROMPT),
  help = STRING_TOKEN(STRING_HELP),
  minsize = 5,
  maxsize = 10,
  warningif
    prompt = STRING_TOKEN(WARNING_IF_PROMPT),
    toupper(pushthis) == stringref(STRING_TOKEN(TEST_STRING))
  endif;
endstring;
```

And this code outputs warning only on "edkii" input:
```
string
  name = StringQuestion,
  varid = FormData.StringValue,
  prompt = STRING_TOKEN(STRING_PROMPT),
  help = STRING_TOKEN(STRING_HELP),
  minsize = 5,
  maxsize = 10,
  warningif
    prompt = STRING_TOKEN(WARNING_IF_PROMPT),
    pushthis == tolower(stringref(STRING_TOKEN(TEST_STRING)))
  endif;
endstring;
```

# length

https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.6.1-length

The `length` keyword help get length of a buffer or a string. It is encoded with the `EFI_IFR_LENGTH_OP = 0x56` opcode.

For example here the code suppressed only if the string question data is a string exactly of 7 symbols:
```
suppressif length(questionref(StringQuestion)) == 7;
  ...
endif;
```

# More advanced string builtins


| keyword | IFR opcode | link |
| --------| -----------| ---- |
| `catenate` | `EFI_IFR_CATENATE` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.1-catenate |
| `match` | `EFI_IFR_MATCH` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.2-match |
| `match2` | `EFI_IFR_MATCH2` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.9-match2 |
| `find` | `EFI_IFR_FIND` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.7.2-find |
| `mid` |	`EFI_IFR_MID` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.7.3-mid |
| `token` | `EFI_IFR_TOKEN` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.7.4-tok |
| `span` | `EFI_IFR_SPAN` | https://edk2-docs.gitbook.io/edk-ii-vfr-specification/2_vfr_description_in_bnf/212_vfr_expression_statement_definition#2.12.11.7.5-span |