How do I deal with special characters like in my regex

Question

I want to match a regular expression special character                 I tried   x  lt -  a b  grepl      x     Error  invalid regular expression      reason  Missing         Equivalently stringr  str detect x       or stringi  stri detect regex x          Doubling the value to escape it doesn t work   grepl       x     Error  invalid regular expression       reason  Missing        Neither does using a backslash   grepl       x     Error       is an unrecognized escape in character string starting         How do I match special characters     Some special cases of this in questions that are old and well written enough for it to be cheeky to close as duplicates of this  Escaped Periods In R Regular Expressions How to escape a question mark in R  escaping pipe   quot   quot   in a regex

User · Answer

I think the easiest way to match the characters like

\^$.?*|+()[

are using character classes from within R. Consider the following to clean column headers from a data file, which could contain spaces, and punctuation characters:

> library(stringr)
> colnames(order_table) <- str_replace_all(colnames(order_table),"[:punct:]|[:space:]","")

This approach allows us to string character classes to match punctation characters, in addition to whitespace characters, something you would normally have to escape with \\ to detect. You can learn more about the character classes at this cheatsheet below, and you can also type in ?regexp to see more info about this.

https://www.rstudio.com/wp-content/uploads/2016/09/RegExCheatsheet.pdf

User · Answer

Escape with a double backslash  R treats backslashes as escape values for character constants       and so do regular expressions  Hence the need for two backslashes when supplying a character argument for a pattern  The first one isn t actually a character  but rather it makes the second one into a character   You can see how they are processed using cat   y  lt -  double quote      tab   t  newline   n  unicode point   u20AC  print y      1   double quote      tab   t  newline   n  unicode point       cat y     double quote     tab       newline        unicode point        Further reading  Escaping a backslash with a backslash in R produces 2 backslashes in a string  not 1    To use special characters in a regular expression the simplest method is usually to escape them with a backslash  but as noted above  the backslash itself needs to be escaped   grepl         a b       1  TRUE   To match backslashes  you need to double escape  resulting in four backslashes   grepl         c  a  b    a nb        1   TRUE FALSE   The rebus package contains constants for each of the special characters to save you mistyping slashes   library rebus  OPEN BRACKET     1        BACKSLASH     1           For more examples see    SpecialCharacters   Your problem can be solved this way   library rebus  grepl OPEN BRACKET   a b     Form a character class  You can also wrap the special characters in square brackets to form a character class   grepl         a b       1  TRUE   Two of the special characters have special meaning inside character classes    and     Backslash still needs to be escaped even if it is inside a character class   grepl           c  a  b    a nb        1   TRUE FALSE   Caret only needs to be escaped if it is directly after the opening square bracket   grepl          a b      matches spaces as well      1  TRUE grepl           a b        1  TRUE   rebus also lets you form a character class   char class          lt regex gt        Use a pre-existing character class  If you want to match all punctuation  you can use the   punct   character class   grepl     punct      c                                          1  TRUE TRUE TRUE TRUE TRUE TRUE TRUE   stringi maps this to the Unicode General Category for punctuation  so its behaviour is slightly different   stri detect regex c                                          punct          1   TRUE  TRUE  TRUE  TRUE  TRUE FALSE FALSE   You can also use the cross-platform syntax for accessing a UGC   stri detect regex c                                         p P        1   TRUE  TRUE  TRUE  TRUE  TRUE FALSE FALSE   Use  Q  E escapes  Placing characters between   Q and   E makes the regular expression engine treat them literally rather than as regular expressions   grepl    Q   E    a b       1  TRUE   rebus lets you write literal blocks of regular expressions   literal          lt regex gt   Q  E   Don t use regular expressions  Regular expressions are not always the answer   If you want to match a fixed string then you can do  for example   grepl       a b   fixed   TRUE  stringr  str detect  a b   fixed       stringi  stri detect fixed  a b

[regex] How do I deal with special characters like \^$.?*|+()[{ in my regex?

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