Appendix A: Regular Expressions
Introduction
Redax Enterprise Server includes the ability to search PDF documents for matches to regular expressions. A regular expression is a flexible pattern used to match (describe) a set of characters.
Simple Examples
A period matches any character.
.at
matches any three-character string ending with “at”, including “hat”, “cat” and “bat”.
Brackets are used for a set of available characters, called a character class. A dash can indicate a range of numbers or characters.
[hc]at
matches “hat” and “cat”.
[c-h]at
matches “cat”, “eat”, “fat” and “hat”.
a pipe “|” symbol is used for “or”. Parentheses are used to show grouping.
(19|20)th
matches “19th” and “20th”: the numbers 19 or 20 followed by “th”.
A question mark makes the previous item optional.
Mrs?\.
matches “Mr.” and “Mrs.” The backslash is added to escape the period special character, that is, to make the period match an ordinary period instead of having the special “match-any-style-character” property described above.
Braces indicate a repeat of the previous item.
a{2,4}
matches “aa”, “aaa” and “aaaa”.
An asterisk matches the previous item zero or more times.
a*
matches “a”, “aa” and “aaa” . . ., and the empty string
A plus matches the previous item 1 or more times.
a+
matches “a”, “aa”, “aaa” and “aaaa” . . .
Sample Regular Expression
From the sample file sample_regex_list.txt, the following two regular expressions (together) will find dates from 1900 to 2049.
19[0-9]{2}
matches 1900-1999: the number 19 followed by a digit from 0 to 9 twice
20[0-4][0-9]
matches 2000-2049: the number 20 followed by a digit from 0 to 4 and a digit from 0 to 9
Regular Expression Basic Syntax Reference
| Character | Description | Example |
|---|---|---|
| Any character except [ \ ^ $ . | ? * + ( ) |
All characters except the listed special characters match a single instance of themselves. { and } are literal characters, unless they’re part of a valid regular expression token (e.g., the {n} quantifier). | a matches a |
| \ (backslash) followed by any of [ \ ^ $ . | ? * + ( ) { } |
A backslash escapes special characters to suppress their special meaning. | \+ matches + |
| \xFF where FF are two hexadecimal digits | Matches the character with the specified ASCII/ANSI value, which depends on the code page used. Can be used in character classes. | \xA9 matches © when using the Latin-1 code page. |
| \t | Match a tab character. Can be used in character classes. | |
| [ (opening square bracket) | Starts a character class. A character class matches a single character out of all the possibilities offered by the character class. Inside a character class, different rules apply. The rules in this section are only valid inside character classes. The rules outside this section are not valid in character classes, except for a few character escapes that are indicated with “can be used inside character classes”. | |
| Any character except ^ – ] \ add that character to the possible matches for the character class. | All characters except the listed special characters. | [abc] matches a, b or c |
| \ (backslash) followed by any of ^ – ] \ |
A backslash escapes special characters to suppress their special meaning. | [\^\]] matches ^ or ] |
| – (hyphen) except immediately after the opening [ | Specifies a range of characters. (Specifies a hyphen if placed immediately after the opening square bracket.) | [a-zA-Z0-9] matches any letter or digit |
| ^ (caret) immediately after the opening [ | Negates the character class, causing it to match a single character not listed in the character class. (Specifies a caret if placed anywhere except after the opening square bracket.) | [^a-d] matches x (any character except a, b, c or d) |
| \d, \w and \s | Shorthand character classes matching digits, word characters (letters, digits, and underscores), and whitespace (spaces, tabs, and line breaks). Can be used inside and outside character classes. | [\d\s] matches a character that is a digit or whitespace |
| \D, \W and \S | Negated versions of the above. Should be used only outside character classes. (Can be used inside, but that is confusing.) | \D matches a character that is not a digit |
| [\b] | Inside a character class, \b is a backspace character. | [\b\t] matches a backspace or tab character |
| . (dot) | Matches any single character except line break characters \r and \n. Most regex flavors have an option to make the dot match line break characters too. | . matches x or (almost) any other character |
| \Z | Matches at the end of the string the regex pattern is applied to. Matches a position rather than a character. Never matches before line breaks, except for the very last line break if the string ends with a line break. | .\Z matches f in abc\ndef |
| \z | Matches at the end of the string the regex pattern is applied to. Matches a position rather than a character. Never matches before line breaks. | .\z matches f in abc\ndef |
| \b | Matches at the position between a word character (anything matched by \w) and a non-word character (anything matched by [^\w] or \W) as well as at the start and/or end of the string if the first and/or last characters in the string are word characters. | .\b matches c in abc |
| \B | Matches at the position between two word characters (i.e., the position between \w\w) as well as at the position between two non-word characters (i.e., \W\W). | \B.\B matches b in abc |
| | (pipe) | Causes the regex engine to match either the part on the left side, or the part on the right side. Can be strung together into a series of options. | abc|def|xyz matches abc, def or xyz |
| | (pipe) | The pipe has the lowest precedence of all operators. It instructs the engine to alternate part of the regular expression. | abc(def|xyz) matches abcdef or abcxyz |
| ? (question mark) | Makes the preceding item optional. Greedy, so the optional item is included in the match if possible. | abc? matches abc or ab |
| ?? | Makes the preceding item optional. Lazy, so the optional item is excluded from the match if possible. This construct is often excluded from documentation because of its limited use. | abc?? matches ab or abc |
| * (star) | Repeats the previous item zero or more times. Greedy, so as many items as possible will be matched before trying permutations with less matches of the preceding item, up to the point where the preceding item is not matched at all. | “.*” matches “def” “ghi” in abc “def” “ghi” jkl |
| *? (lazy star) | Repeats the previous item zero or more times. Lazy, so the engine first attempts to skip the previous item, before trying permutations with ever increasing matches of the preceding item. | “.*?” matches “def” in abc “def” “ghi” jkl |
| + (plus) | Repeats the previous item once or more. Greedy, so as many items as possible will be matched before trying permutations with less matches of the preceding item, up to the point where the preceding item is matched only once. | “.+” matches “def” “ghi” in abc “def” “ghi” jkl |
| +? (lazy plus) | Repeats the previous item once or more. Lazy, so the engine first matches the previous item only once, before trying permutations with ever increasing matches of the preceding item. | “.+?” matches “def” in abc “def” “ghi” jkl |
| {n} where n is an integer >= 1 | Repeats the previous item exactly n times. | a{3} matches aaa |
| {n,m} where n >= 0 and m >= n | Repeats the previous item between n and m times. Greedy, so repeating m times is tried before reducing the repetition to n times. | a{2,4} matches aaaa, aaa or aa |
| {n,m}? where n >= 0 and m >= n | Repeats the previous item between n and m times. Lazy, so repeating n times is tried before increasing the repetition to m times. | a{2,4}? matches aa, aaa or aaaa |
| {n,} where n >= 0 | Repeats the previous item at least n times. Greedy, so as many items as possible will be matched before trying permutations with less matches of the preceding item, up to the point where the preceding item is matched only n times. | a{2,} matches aaaaa in aaaaa |
| {n,}? where n >= 0 | Repeats the previous item n or more times. Lazy, so the engine first matches the previous item n times, before trying permutations with ever increasing matches of the preceding item. | a{2,}? matches aa in aaaaa |
Other Resources
Learn more about regular expressions from Wikipedia.
http://en.wikipedia.org/wiki/Regular_expression
The International Components for Unicode (ICU) provides an excellent User Guide for regular expressions.
http://userguide.icu-project.org/strings/regexp
RegExLib is a regular expressions catalog.
RegexBuddy is a Windows utility that makes it easy for non-technical users to develop Regular Expressions.