# Regular Expressions

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Regular expressions describe patterns in strings that can be used in order to determine whether a given pattern occurs in a text or not. In TreeSize, regular expressions can be used to find specific files and / or folders that match the criteria specified by regular expressions

The following table shows some of the most used syntax and provides a few examples:

Expression | Syntax | Description | Example
Any character | . | Matches any single character except a line break. | a.o matches "aro" in "around" and "abo" in "about" but not "acro" in "across".
Zero or more | * | Matches zero or more occurrences of the preceding expression, and makes all possible matches. | a*b matches "b" in "bat" and "ab" in "about". e.*e matches the word "enterprise".
One or more | + | Matches at least one occurrence of the preceding expression. | ac+ matches words that contain the letter "a" and at least one instance of "c", such as "race", and "ace". a.+s matches the word "access".
Start of string | ^ | Matches the start of a string | ^[0-9] matches strings that start with a digit.
End of string | $ | Matches the end of a string | exe$ matches strings that end with "exe".
Beginning of word | [[:<:]] | Matches only when a word starts at this point in the text. | [[:<:]]in matches words such as "inside" and "into" that begin with the letters "in".
End of word | [[:>:]] | Matches only when a word ends at this point in the text. | ss[[:>:]] matches words such as "across" and "loss" that end with the letters "ss".
Any one character in the set | [] | Matches any one of the characters in the []. To specify a range of characters, list the starting and ending characters separated by a dash (-), as in [a-z]. | be[n-t] matches "bet" in "between", "ben" in "beneath", and "bes" in "beside" but not "bel" in "below".
Any one character not in the set | [^...] | Matches any character that is not in the set of characters that follows the ^. | be[^n-t] matches "bef" in "before", "beh" in "behind", and "bel" in "below", but not "ben" in "beneath".
Or | | | Matches either the expression before or the one after the OR symbol (|). Mostly used in a group. | (sponge|mud) matches "sponge bath" and "mud bath.
Escape character | \ | Matches the character that follows the backslash (\) as a literal. This lets you find the characters that are used in regular expression notation, such as { and ^. | \^ searches for the ^ character.
Repeat n times | {n} | Matches n occurrences of the preceding expression. | [0-9]{4} matches any 4-digit sequence.
Grouping | () | Lets you group a set of expressions together. If you want to search for two different expressions in a single search, you can use the Grouping expression to combine them. | If you want to search for [a-z][1-3] or [0-9][a-z], you would combine them: ([a-z][1-3])|([0-9][a-z]).

More examples:

Regular Expression | Use Case
[0-9] or \d | Find all files/folders with at least one digit in its name.
a|b | Find all files/folders containing "a" or "b" in their name.
[^(A-Za-z)] | Find all files/folders containing at least one character in their name that is not in the range A-Z or a-z.
^E[0-9]{7}$ | Find all files/folders which start with an "E" followed by exactly 7 digits.
[A-Za-z]:\\([^\\]+\\){2,4}[^\\]+$ | Find all files/folders with a folder depth of at least 2 and at most 4.
[^\x00-\x7F] | Find all files/folders with invalid ASCII characters.
[^\P{C}] | Find all files/folders with Unicode characters which cannot be printed.
[\xA0] | Find all file/Folder names that contain the non-breakable space character (Unicode NOBR, U+00A0) instead of a normal space character.
[~\"#%&\*\:<>\?\/\\{|}] | Find all file and folder names, that contain characters which are invalid on SharePoint servers.
^\s+.* | Find all files and folders with a leading space.
\s+(\.[^.]+)$ | Find files with an extension that have a trailing space at the end of their name.
.*\s+$ | Find folders with a trailing space at the end of their name

Further information and additional examples can be found here.

A description of all special characters that can be used with regular expression can be found here.

The following tools can assist in forming regular expressions:

https://regex101.com/ (online)

http://regexpal.com/ (online)

http://sourceforge.net/projects/regexpeditor/ (download)

http://sourceforge.net/projects/regextester/ (download)

http://sourceforge.net/projects/regaxe/ (download)