Regular Expressions

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:

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.

When using a regex in a PhixFlow expression, strings must be contained inside quotes, for example "match this text", except: 

  • in PhixFlow variables that contain strings
  • in functions that return strings.

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.

replaceAll(listToCheck, “##” + “[0-9]{12}” + “##”, “barcode discontinued”)

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

RegexPurpose
"inputFile_[0-9]{3}\\.txt"Match file names with the format: 
inputFile_NNN.txt    e.g. inputFile_034.txt
where N are digits.
"inputFile_[0-9]{3}\\.txt"Match file names with the format:
inputFile_NNN.txt    eg. inputFile_034.txt
where N are digits.
"inputFile_" + $fileSeq + "\\.txt"Match file names with the format:
inputFile_$fileSeq.txt   e.g. inputFile_034.txt
where $fileSeq="034"

Regex Arguments in Expressions

if(!matches(value,"[\\p{Alnum}\\p{Space}]+"),error('Please only enter alpha numeric characters')

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.

$xml = replaceAll($xml, "<sc_tower_torqueing/type><id.*?<name>", "<type>"),
$xml = replaceAll($xml, "</name></sc_tower_torqueing/type>", "</type>"),

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.

UseTo escapeFor example
2 backslashes\\

. () [] {}

 * ^ + | ?

\\.
\\]
\\^

3 backslashes\\\

a backslash

quote marks "

\\\\

\\\"


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

replaceAll(JSON Code,"[^\\p{Space}0-9A-Za-z!:\\\"%&\\[*()\\],-/_\\\\{}\\.]","")

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 ""

A double backslash \\ can also make an alphanumeric character a construct. For example, \\n means newline; see Characters below. Escaping an alphabetic character that does not form construct causes an error.

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:

1    

((A)(B(C)))

2    

(A)

3    

(B(C))

4    

(C)

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.

Sections on this page

Characters

ConstructMatches
xThe character x
\\\\The backslash character
\\/The forward slash character
\\\"The double quote character
\\0nThe character with octal value 0n (0 <= n <= 7)
\\0nnThe character with octal value 0nn (0 <= n <= 7)
\\0mnnThe character with octal value 0mnn (0 <= m <= 3, 0 <= n <= 7)
\\xhhThe character with hexadecimal value 0xhh
\uhhhhThe character with hexadecimal value 0xhhhh
\\tThe tab character ('\u0009')
\\nThe newline (line feed) character ('\u000A')
\\rThe carriage-return character ('\u000D')
\\fThe form-feed character ('\u000C')
\\aThe alert (bell) character ('\u0007')
\\eThe escape character ('\u001B')
\\cxThe control character corresponding to x

Character Classes

ConstructMatches
[abc]a, b, or c (simple class)
[^abc]Any character except a, b, or c (negation)
[a-zA-Z]a through z or A through Z, inclusive (range)
[a-d[m-p]]a through d, or m through p: [a-dm-p] (union)
[a-z&&[def]]d, e, or f (intersection)
[a-z&&[^bc]]a through z, except for b and c: [ad-z] (subtraction)
[a-z&&[^m-p]]a through z, and not m through p: [a-lq-z](subtraction)
Predefined
.Any character wildcard
This may or may not match line terminators
\\dA digit: [0-9]
\\DA non-digit: [^0-9]
\\sA whitespace character: [ \\t\\n\\x0B\\f\\r]
\\SA non-whitespace character: [^\\s]
\\wA word character: [a-zA-Z_0-9]
\\WA non-word character: [^\\w]

Line Terminators 

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

ConstructIndicates
\\nnewline (line feed)
\\rcarriage-return 
\\r\\ncarriage-return followed immediately by a newline 
\\u0085next-line
\\u2028line-separator
\\u2029paragraph-separator

POSIX Classes

(US-ASCII only)

ConstructMatches
\\p{Lower}A lower-case alphabetic character: [a-z]
\\p{Upper}An upper-case alphabetic character:[A-Z]
\\p{ASCII}All ASCII:[\\x00-\\x7F]
\\p{Alpha}An alphabetic character:[\\p{Lower}\\p{Upper}]
\\p{Digit}A decimal digit: [0-9]
\\p{Alnum}An alphanumeric character:[\\p{Alpha}\\p{Digit}]
\\p{Punct}Punctuation: One of !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
\\p{Graph}A visible character: [\\p{Alnum}\\p{Punct}]
\\p{Print}A printable character: [\\p{Graph}]
\\p{Blank}A space or a tab: [ \\t]

\\p{Cntrl}

A control character: [\\x00-\\x1F\\x7F]
\\p{XDigit}A hexadecimal digit: [0-9a-fA-F]
\\p{Space}A whitespace character: [ \\t\\n\\x0B\\f\\r]

Unicode Classes

ConstructMatches
\\p{InGreek}A character in the Greek block (simple block)
\\p{Lu}An uppercase letter (simple category)

\\p{Sc}

A currency symbol
\\P{InGreek}Any character except one in the Greek block (negation)
[\\p{L}&&[^\\p{Lu}]] Any letter except an uppercase letter (subtraction)

Unicode Characters

ConstructMatches
\x{20ac}The Euro symbol €
\x{1f914}An emoji character 🤔 

Boundaries

ConstructMatches
^The beginning of a line
$The end of a line
\\bA word boundary
\\BA non-word boundary
\\AThe beginning of the input
\\GThe end of the previous match
\\ZThe end of the input but for the final, if any
\\zThe end of the input

Back References

ConstructMatches
$nWhatever the nth capturing group matched; see Groups and Capturing, above.

Quantifiers

ConstructMatches
Greedy
longest possible match

Reluctant (lazy)
shortest possible match

Possessive


X?X??X?+X, once or not at all
X*X*?X*+X, zero or more times
X+X+?X++X, one or more times
X{n}X{n}?X{n}+X, exactly n times
X{n,}X{n,}?X{n,}+X, at least n times
X{n,m}X{n,m}?X{n,m}+X, at least n but not more than m times

Logical Operators

ConstructMatches
XYX followed by Y
X|YEither X or Y
(X)X, as a capturing group; see Groups and Capturing, above.

Special Constructs

(Non-capturing)

ConstructMatches
(?:X)X, as a non-capturing group
(?idmsux-idmsux) Nothing, but turns match flags on - off
(?idmsux-idmsux:X)X, as a non-capturing group with the given flags on - off; see Groups and Capturing, above.
(?=X)X, via zero-width positive lookahead
(?!X)X, via zero-width negative lookahead
(?<=X)X, via zero-width positive lookbehind
(?<!X)X, via zero-width negative lookbehind
(?>X)X, as an independent, non-capturing group

Quotation

ConstructMatches
\\Nothing, but quotes the following character
\\QNothing, but quotes all characters until \\E
\\ENothing, but ends quoting started by \\Q