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Literal Characters

The most basic regular expression consists of a single literal character, e.g.: a. It will match the first occurrence of that character in the string. If the string is Jack is a boy, it will match the a after the J. The fact that this a is in the middle of the word does not matter to the regex engine. If it matters to you, you will need to tell that to the regex engine by using word boundaries. We will get to that later.

This regex can match the second a too. It will only do so when you tell the regex engine to start searching through the string after the first match. In a text editor, you can do so by using its "Find Next" or "Search Forward" function. In a programming language, there is usually a separate function that you can call to continue searching through the string after the previous match.

Similarly, the regex cat will match cat in About cats and dogs. This regular expression consists of a series of three literal characters. This is like saying to the regex engine: find a c, immediately followed by an a, immediately followed by a t.

Note that regex engines are case sensitive by default. cat does not match Cat, unless you tell the regex engine to ignore differences in case.

Special Characters

Because we want to do more than simply search for literal pieces of text, we need to reserve certain characters for special use. In the regex flavors discussed in this tutorial, there are 11 characters with special meanings: the opening square bracket [, the backslash \, the caret ^, the dollar sign $, the period or dot ., the vertical bar or pipe symbol |, the question mark ?, the asterisk or star *, the plus sign +, the opening round bracket ( and the closing round bracket ). These special characters are often called "metacharacters".

If you want to use any of these characters as a literal in a regex, you need to escape them with a backslash. If you want to match 1+1=2, the correct regex is 1\+1=2. Otherwise, the plus sign will have a special meaning.

Note that 1+1=2, with the backslash omitted, is a valid regex. So you will not get an error message. But it will not match 1+1=2. It would match 111=2 in 123+111=234, due to the special meaning of the plus character.

If you forget to escape a special character where its use is not allowed, such as in +1, then you will get an error message.

Most regular expression flavors treat the brace { as a literal character, unless it is part of a repetition operator like {1,3}. So you generally do not need to escape it with a backslash, though you can do so if you want. An exception to this rule is the java.util.regex package: it requires all literal braces to be escaped.

All other characters should not be escaped with a backslash. That is because the backslash is also a special character. The backslash in combination with a literal character can create a regex token with a special meaning. E.g. \d will match a single digit from 0 to 9.

Escaping a single metacharacter with a backslash works in all regular expression flavors. Many flavors also support the \Q...\E escape sequence. All the characters between the \Q and the \E are interpreted as literal characters. E.g. \Q*\d+*\E matches the literal text *\d+*. The \E may be omitted at the end of the regex, so \Q*\d+* is the same as \Q*\d+*\E. This syntax is supported by the JGsoft engine, Perl, PCRE and Java, both inside and outside character classes. However, in Java, this feature does not work correctly in JDK 1.4 and 1.5 when used in a character class or followed by a quantifier.

Special Characters and Programming Languages

If you are a programmer, you may be surprised that characters like the single quote and double quote are not special characters. That is correct. When using a regular expression or grep tool like PowerGREP or the search function of a text editor like EditPad Pro, you should not escape or repeat the quote characters like you do in a programming language.

In your source code, you have to keep in mind which characters get special treatment inside strings by your programming language. That is because those characters will be processed by the compiler, before the regex library sees the string. So the regex 1\+1=2 must be written as "1\\+1=2" in C++ code. The C++ compiler will turn the escaped backslash in the source code into a single backslash in the string that is passed on to the regex library. To match c:\temp, you need to use the regex c:\\temp. As a string in C++ source code, this regex becomes "c:\\\\temp". Four backslashes to match a single one indeed.

See the tools and languages section of this website for more information on how to use regular expressions in various programming languages.

Non-Printable Characters

You can use special character sequences to put non-printable characters in your regular expression. Use \t to match a tab character (ASCII 0x09), \r for carriage return (0x0D) and \n for line feed (0x0A). More exotic non-printables are \a (bell, 0x07), \e (escape, 0x1B), \f (form feed, 0x0C) and \v (vertical tab, 0x0B). Remember that Windows text files use \r\n to terminate lines, while UNIX text files use \n.

You can include any character in your regular expression if you know its hexadecimal ASCII or ANSI code for the character set that you are working with. In the Latin-1 character set, the copyright symbol is character 0xA9. So to search for the copyright symbol, you can use \xA9. Another way to search for a tab is to use \x09. Note that the leading zero is required.

Most regex flavors also support the tokens \cA through \cZ to insert ASCII control characters. The letter after the backslash is always a lowercase c. The second letter is an uppercase letter A through Z, to indicate Control+A through Control+Z. These are equivalent to \x01 through \x1A (26 decimal). E.g. \cM matches a carriage return, just like \r and \x0D. In XML Schema regular expressions, \c is a shorthand character class that matches any character allowed in an XML name.

If your regular expression engine supports Unicode, use \uFFFF rather than \xFF to insert a Unicode character. The euro currency sign occupies code point 0x20AC. If you cannot type it on your keyboard, you can insert it into a regular expression with \u20AC.

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