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# Regular expressions

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. A list of special characters are supported.

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.

## Tools for creating 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)