This page is for anyone writing expressions in which you need to match string patterns.

Overview

When you are writing expressions you may need to find patterns within strings or arrays. To do this, you can add a regular expression (commonly abbreviated to regex) as an argument to your expression. Some functions that take a regex as an argument are replaceAll , replaceFirst and matches.

If you are unfamiliar with regular expressions, there are many resources on the internet that can get you started, such as:

• https://en.wikipedia.org/wiki/Regular_expression

• https://www.regular-expressions.info/quickstart.html

This page provides information and examples of common uses of regular expressions in PhixFlow. It also includes tables showing Character Constructs and what they mean.

Examples

Finding Barcodes

A bigstring of text contains barcodes that need to be replaced with something else. Every barcode has a different number, but barcodes have the same pattern. An example barcode is the string ##167891546325##. We can’t match an exact sequence as every barcode is different. However we can use a regex to match the pattern.

The regex “##” + “[0-9]{12}” + “##” matches the pattern of:

• two hashes

• followed by 12 numbers

• followed by two hashes.

Finally, expression replaceAll() replaces any matches to the text barcode discontinued.

Finding File Names

Regex Arguments in Expressions

Checks whether any characters are not (!) alphanumeric characters or spaces. If any are not, matches() will evaluate to 1, and the error message will be presented. The + is a greedy quantifier, so it can match more than once. This means value can contain as many alphanumeric characters or spaces as it likes, so long as it does not contain anything else.

The first line matches text with the pattern:

•  sc_tower_torqueing/type><id.

• followed by

  • any character, indicated by the wildcard * 

  • however many times it appears, indicated by ?

• followed by the text <name> to finish.

Any matching pattern is changed to the text <type>. 

The second line matches the exact text </name></sc_tower_torqueing/type> and changes any occurrence to “</type>". As the exact text is known, this does not require a regex.

Escaping Special Characters 

In a regex, some characters, called meta-characters, have special meanings. For example quote marks “ mark the the start and end of a string; see Regular expression metacharacters for a full list and their functions. To use a meta-character as if it is a normal character, use an escape character so that the regex can ignore its special meaning.

For example, the following regex shows some characters are escaped and some are not.

This regex checks text to make sure it does not contain any unusual characters by defining a list of allowed characters:

• paragraph end marks: \\p 

• spaces: {​Space}

• numbers 0-9: ​0-9

• uppercase or lowercase alphabetic characters: A-Za-z

• other characters: ! : % & * ( ) , - / _ { }

• special characters, preceded by backslashes to escape them: " [ ] \  .

Characters with special meaning are:

• quotes and square brackets: “[ and ]"indicate the start and the end of the string 

• caret ^ means NOT, so match anything that is NOT in the following list.

This regex ignores any of the allowed characters. If it finds any character not in the list, it replaces them with nothing, indicated by "". 

For information about escaping characters in PhixFlow expressions, rather than in a regex, see Text Expressions and Escape Characters.

Groups and Capturing 

Capturing groups are numbered by counting their opening parentheses from left to right. For example,  ((A)(B(C))) contains 4 groups:

Group zero always stands for the entire regex.

During a match, each sub-sequence of the input sequence that matches a group is saved. The captured sub-sequence may be used later in the regex, via a back reference. In PhixFlow, a back reference has the format $n, where n is the number of the capturing group; replaceFirst for some examples.

The captured input associated with a group is always the sub-sequence that the group most recently matched. If a group is evaluated a second time because of a quantifier, then any previously captured value is retained if the second evaluation fails. For example, matching the string "aba" against the regex (a(b)?)+ leaves group two set to "b". All captured input is discarded at the beginning of each match.

Groups beginning with (? are pure, non-capturing groups that do not capture text and do not count towards the group total.

Character Constructs 

The following table shows the different characters and what they match.

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Characters

Character Classes

Line Terminators 

A one- or two-character sequence that marks the end of a line of the input character sequence.

POSIX Classes

(US-ASCII only)

Unicode Classes

Unicode Characters

Boundaries

Back References

Quantifiers