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like

like, operator

The binary candidate like pattern pure operator returns true if the text candidate matches the pattern described by the text pattern. Patterns interpret * to mean zero, one or more characters, and ? to mean exactly one character.

The unary like pattern is used as a pattern in a match statement, where it is translated to the binary form with the match expression as left operand.

Examples

As an expression

table Test = with 
  [| as Text,    as Pattern |]
  [| "ABCDE",    "AB?DE"    |]
  [| "ABCDE",    "A?E"      |]
  [| "ABCDE",    "A*E"      |]
  [| "ABCDE",    "ABCD*E"   |]
  [| "ABCDECDE", "A*C*E"    |]
  [| "ABCDE",    "*CDE"     |]
  [| "ABCDE",    "ABC*"     |]
  [| "ABCDE",    "*C*"      |]

show table "Like" with 
  Test.Text
  Test.Pattern
  Test.Text like Test.Pattern

Produces the following output:

Text	Pattern	Test.Text like Test.Pattern
ABCDE	AB?DE	true	’?’ matches C
ABCDE	A?E	false	’?’ cannot match BCD
ABCDE	A*E	true	’’ matches BCD*
ABCDE	ABCD*E	true	’’ matches zero characters*
ABCDECDE	ACE	true	’’ & ‘’ match BCDE & D, or B & DECD
ABCDE	*CDE	true	’’ matches AB*
ABCDE	ABC*	true	’’ matches DE*
ABCDE	C	true	’’ & ‘’ match AB & DE

As a match pattern

table Test with 
  [| as Text |]
  [| "A"     |]
  [| "AB"    |]
  [| "CDE"   |]
  [| "EDCB"  |]
  [| "XEY"   |]
  [| "E"     |]

Test.Matched = match Test.Text with 
  like "A*" -> "A*"
  like "*B" -> "*B"
  like "*D*" -> "*D*"
  like "?E?" -> "?E?"
  .. -> "Other"

show table "Like" with 
  Test.Text
  Test.Matched

Produces the following output:

Text	Matched
A	A*	’’ matches zero characters*
AB	A*	’’ matches B, does not reach second line of match*
CDE	D	’’ & ‘’ match C and E
EDCB	*B	’’ matches EDC, does not reach third line of match*
XEY	?E?	’?’ and ‘?’ match X and Y
E	Other

Remarks

Forbidden characters

This operator is similar to the SQL LIKE operator, but does not use the the same special characters: SQL uses % instead of * to match an arbitrary number of characters, and _ instead of ? to match exactly one character. To avoid confusion, the characters % and _ are not allowed inside a like pattern.

To allow for future expansion, the characters \, [ and ( are also not allowed inside a like pattern.

Escaping characters

To use a forbidden character inside a like pattern, or to represent the * and ? characters without their special meaning, they must be escaped by surrounding them with []. Only one character is allowed between the square brackets, and that character is expected to be matched verbatim.

show summary "Escaping" with
  "ABC" like "A*"     // true
  "ABC" like "A[*]"   // false
  "A*"  like "A[*]"   // true
  "$"   like "[$]"    // error: $ does not need to be escaped. 
  "[$]" like "[[]$]"  // true
  "_"   like "[_]"    // true
  "_"   like "_"      // error: _ needs to be escaped.
  "__"  like "[__]"   // error: only one character allowed in [ ]
  "__"  like "[_][_]" // true

Use function escapelike to escape the contents of a text variable so that it can be safely included in a like pattern.

Optimization

Since there is a cost to parsing arbitrary patterns at runtime, Envision automatically detects if a compile-time constant pattern can be converted to more optimized function call. For example:

A like "*BCD*" is a synonym for contains(A, "BCD")
A like "BCD*" is a synonym for startsWith(A, "BCD")
A like "*BCD" is a synonym for endsWith(A, "BCD")
A like "B*C" is a synonym for startsWith(A, "B") and endsWith(A, "C")
A like "??" is a synonym for strlen(A) == 2

…and so on. Therefore, it is advisable to use like instead of text functions whenever that ends in more readable code.

Errors

When a forbidden character (_, %, (, \, [) is used without being escaped:

`like`: character “%” must be escaped as “[%]”

When a character escape block (starting with [ and ending with ]) contains more than one, or less than one, characters:

`like`: use “[” and “]” to escape exactly one character at a time.

When a character escape block contains a character that does not need to be escaped:

`like`: in “[A]”, character “A” does not need to be escaped. Did you mean “A” or “[[]A]” ?