To go beyond matching literal text, regex engines reserve certain characters for special functions. These are known as metacharacters. The following characters have special meanings in most regex flavors discussed in this tutorial:

[ \ ^ $ . | ? * + ( )

If you need to use any of these characters as literals in your regex, you must escape them with a backslash (\). For instance, to match "1+1=2", you would write the regex as:

1\+1=2

Without the backslash, the plus sign would be interpreted as a quantifier, causing unexpected behavior. For example, the regex «1+1=2» would match "111=2" in the string "123+111=234" because the plus sign is interpreted as "one or more of the preceding characters."

Escaping Special Characters

To escape a metacharacter, simply prepend it with a backslash (). For example:

• «.» matches a literal dot.

• «*» matches a literal asterisk.

• «+» matches a literal plus sign.

Most regex flavors also support the \Q...\E escape sequence. This treats everything between \Q and \E as literal characters. For example:

\Q*\d+*\E

This pattern matches the literal text "\d+". If the \E is omitted at the end, it is assumed. This syntax is supported by many engines, including Perl, PCRE, Java, and JGsoft, but it may have quirks in older Java versions.

Special Characters in Programming Languages

If you're a programmer, you might expect characters like single and double quotes to be special characters in regex. However, in most regex engines, they are treated as literal characters.

In programming, you must be mindful of characters that your language treats specially within strings. These characters will be processed by the compiler before being passed to the regex engine. For instance:

• To use the regex «1+1=2» in C++ code, you would write it as "1\+1=2". The compiler converts the double backslashes into a single backslash for the regex engine.

• To match a Windows file path like "c:\temp", the regex would be «c:\temp», and in C++ code, it would be written as "c:\\temp".

Refer to the specific language documentation to understand how to handle regex patterns within your code.