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README.md

RegEx Transformation Service

Transforms a source string on basis of the regular expression (regex) search pattern to a defined result string.

The simplest regex is in the form <regex> and transforms the input string on basis of the regex pattern to a result string. A full regex is in the form s/<regex>/<substitution>/g whereat the delimiter s and the regex flag g have a special meaning.

The regular expression in the format s/<regex>/result/g, replaces all occurrences of <regex> in the source string with result. The regular expression in the format s/<regex>/result/ (without g), replaces the first occurrence of <regex> in the source string with result.

If the regular expression contains a capture group defined by (), it returns the captured string. Multiple capture groups can be used to retrieve multiple strings and can be combined as a result string defined in the substitution.

The transformation can be set to be restricted to only match if the input string begins with a character by prepending ^ to the beginning of a pattern or to only match if the input string ends with a specified character by appending $ at the end. So the regex ^I.*b$ only matches when the input string starts with I and ends with b, like in I'm Bob. Both can be used alone or in combination.

The special characters \.[]{}()*+-?^$| have to be escaped when they should be used as literal characters.

Examples

Basic Examples

Input String Regular Expression Output String Explanation
My network does not work. s/work/cast/g "My netcast does not cast." Replaces all matches of the string "work" with the string "cast".
My network does not work. .*(\snot).* " not" Returns only the first match and strips of the rest, "\s" defines a whitespace.
temp=44.0'C temp=(.*?)'C) 44.0 Matches whole string and returns the content of the captcha group (.?).
48312 s/(.{2})(.{3})/$1.$2/g 48.312 Captures 2 and 3 character, returns first capture group adds a dot and the second capture group. This divides by 1000.

Example In Setup

Input String

temp=44.0'C

the regex transformation can be used to extract the value to display it on the label.

.items

String  Temperature_str "Temperature [REGEX(.*=(\\d*.\\d*).*):%s °C]" {...}
Number  Temperature "Temperature [%.1f °C]"

The regex pattern is is defined as follows

  • .* match any character, zero and unlimited times
  • = match the equal sign literally, used to find the position
  • () capture group match
    • \d* match a digit (equal to [0-9]), zero and unlimited times, the backslash has to be escaped see string vs plain
    • . match the dot literally
    • \w* match a word character (equal to [a-zA-Z_0-9]), zero and unlimited times, the backslash has to be escaped see string vs plain
  • .* match any character, zero and unlimited times

The result will be 44.0 and displayed on the label as Temperature 44.0°C. A better solution would be to use the regex on the result from the binding either in a rule or when the binding allows it on the output channel. Thus the value 44.0 would be saved as a number.

.rules

rule "Convert String to Item Number"
  when
    Item Temperature_str changed
 then
    // use the transformation service to retrieve the value
    val newValue = transform("REGEX", ".*=(\\d*.\\d*).*", Temperature_str.state.toString)

    // post the new value to the Number Item
    Temperature.postUpdate( newValue )
 end

Now the resulting Number can also be used in the label to change the color or in a rule as value for comparison.

Differences to plain Regex

The regex is embedded in a string so when double quotes " are used in a regex they need to be escaped \" to keep the string intact. As the escape character of strings is the backslash this has to be escaped additionally. To use a dot as literal in the regex it has to be escape \., but in a string it has to be escaped twice "\\.". The first backslash escapes the second backslash in the string so it can be used in the regex. Using a backslash in a Regex as literal \\ will have this form "\\\\".

Usage as a Profile

The functionality of this TransformationService can be used in a Profile on an ItemChannelLink too. To do so, it can be configured in the .items file as follows:

String <itemName> { channel="<channelUID>"[profile="transform:REGEX", function="<regex>", sourceFormat="<valueFormat>"]}

The regular expression to be executed has to be set in the function parameter. The parameter sourceFormat is optional and can be used to format the input value before the transformation, i.e. %.3f. If omitted the default is %s, so the input value will be put into the transformation without any format changes.

Please note: This profile is a one-way transformation, i.e. only values from a device towards the item are changed, the other direction is left untouched.

Further Reading

  • A full introduction for regular expression is available at W3School.
  • Online validator help to check the syntax of a regex and give information how to design it.
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