v1.1

Character Processing Functions and Operators

String functions and operators provided by MogDB are for concatenating strings with each other, concatenating strings with non-strings, and matching the patterns of strings.

bit_length(string)

Description: Specifies the number of bits occupied by a string.

Return type: int

Example:
```
mogdb=# SELECT bit_length('world');
 bit_length
------------
         40
(1 row)
```
btrim(string text [, characters text])

Description: Removes the longest string consisting only of characters in characters (a space by default) from the start and end of string.

Return type: text

Example:
```
mogdb=# SELECT btrim('sring' , 'ing');
 btrim
-------
 sr
(1 row)
```
char_length(string) or character_length(string)

Description: Number of characters in a string

Return type: int

Example:
```
mogdb=# SELECT char_length('hello');
 char_length
-------------
           5
(1 row)
```
instr(text,text,int,int)

Description: instr(string1,string2,int1,int2) returns the position where string2 has been matched for int2 times starting from int1 in string1. int1 indicates the starting position for matching, and int2 indicates the number of matching times.

Return type: int

Example:
```
mogdb=# SELECT instr( 'abcdabcdabcd', 'bcd', 2, 2 );
 instr
-------
     6
(1 row)
```
lengthb(text/bpchar)

Description: Obtains the number of bytes of a specified string.

Return type: int

Example:
```
mogdb=# SELECT lengthb('hello');
 lengthb
---------
       5
(1 row)
```
left(str text, n int)

Description: Returns first n characters in the string. When n is negative, return all but the last |n| characters.

Return type: text

Example:
```
mogdb=# SELECT left('abcde', 2);
 left
------
 ab
(1 row)
```
length(string bytea, encoding name )

Description: Number of characters in string in the given encoding. The string must be valid in this encoding.

Return type: int

Example:
```
mogdb=# SELECT length('jose', 'UTF8');
 length
--------
      4
(1 row)
```
lpad(string text, length int [, fill text])

Description: Fills up the string to length length by prepending the characters fill (a space by default). If the string is already longer than length then it is truncated (on the right).

Return type: text

Example:
```
mogdb=# SELECT lpad('hi', 5, 'xyza');
 lpad
-------
 xyzhi
(1 row)
```

octet_length(string)

Description: Number of bytes in a string

Return type: int

Example:

mogdb=# SELECT octet_length('jose');
 octet_length
--------------
            4
(1 row)

overlay(string placing string FROM int [for int])

Description: Replaces substring. FROM int indicates the start position of the replacement in the first string. for int indicates the number of characters replaced in the first string.

Return type: text

Example:
```
mogdb=# SELECT overlay('hello' placing 'world' from 2 for 3 );
 overlay
---------
 hworldo
(1 row)
```
position(substring in string)

Description: Specifies the position of a substring. The string is case sensitive.

Return type: int, 0 is returned when the string does not exist.

Example:
```
mogdb=# SELECT position('ing' in 'string');
 position
----------
        4
(1 row)
```

pg_client_encoding()

Description: Current client encoding name

Return type: name

Example:

mogdb=# SELECT pg_client_encoding();
 pg_client_encoding
--------------------
 UTF8
(1 row)

quote_ident(string text)

Description: Returns the given string suitably quoted to be used as an identifier in an SQL statement string (quotation marks are used as required). Quotes are added only if necessary (that is, if the string contains non-identifier characters or would be case-folded). Embedded quotes are properly doubled.

Return type: text

Example:
```
mogdb=# SELECT quote_ident('hello world');
 quote_ident
--------------
 "hello world"
(1 row)
```
quote_literal(string text)

Description: Returns the given string suitably quoted to be used as a string literal in an SQL statement string (quotation marks are used as required).

Return type: text

Example:
```
mogdb=# SELECT quote_literal('hello');
 quote_literal
---------------
 'hello'
(1 row)
```
If command similar to the following exists, text will be escaped.
```
mogdb=# SELECT quote_literal(E'O\'hello');
 quote_literal
---------------
 'O''hello'
(1 row)
```
If command similar to the following exists, backslash will be properly doubled.
```
mogdb=# SELECT quote_literal('O\hello');
 quote_literal
---------------
 E'O\\hello'
(1 row)
```
If the parameter is null, return NULL. If the parameter may be null, you are advised to use quote_nullable.
```
mogdb=# SELECT quote_literal(NULL);
 quote_literal
---------------

(1 row)
```
quote_literal(value anyelement)

Description: Converts the given value to text and then quotes it as a literal.

Return type: text

Example:
```
mogdb=# SELECT quote_literal(42.5);
 quote_literal
---------------
 '42.5'
(1 row)
```
If command similar to the following exists, the given value will be escaped.
```
mogdb=# SELECT quote_literal(E'O\'42.5');
 quote_literal
---------------
 '0''42.5'
(1 row)
```
If command similar to the following exists, backslash will be properly doubled.
```
mogdb=# SELECT quote_literal('O\42.5');
 quote_literal
---------------
 E'O\\42.5'
(1 row)
```

quote_nullable(string text)

Description: Returns the given string suitably quoted to be used as a string literal in an SQL statement string (quotation marks are used as required).

Return type: text

Example:

mogdb=# SELECT quote_nullable('hello');
 quote_nullable
----------------
 'hello'
(1 row)

If command similar to the following exists, text will be escaped.

mogdb=# SELECT quote_nullable(E'O\'hello');
 quote_nullable
----------------
 'O''hello'
(1 row)

If command similar to the following exists, backslash will be properly doubled.

mogdb=# SELECT quote_nullable('O\hello');
 quote_nullable
----------------
 E'O\\hello'
(1 row)

If the parameter is null, return NULL.

mogdb=# SELECT quote_nullable(NULL);
 quote_nullable
----------------
 NULL
(1 row)

quote_nullable(value anyelement)

Description: Converts the given value to text and then quotes it as a literal.

Return type: text

Example:

mogdb=# SELECT quote_nullable(42.5);
 quote_nullable
----------------
 '42.5'
(1 row)

If command similar to the following exists, the given value will be escaped.

mogdb=# SELECT quote_nullable(E'O\'42.5');
 quote_nullable
----------------
 'O''42.5'
(1 row)

If command similar to the following exists, backslash will be properly doubled.

mogdb=# SELECT quote_nullable('O\42.5');
 quote_nullable
----------------
 E'O\\42.5'
(1 row)

If the parameter is null, return NULL.

mogdb=# SELECT quote_nullable(NULL);
 quote_nullable
----------------
 NULL
(1 row)

substring(string [from int] [for int])

Description: Extracts a substring. from int indicates the start position of the truncation. for int indicates the number of characters truncated.

Return type: text

Example:
```
mogdb=# SELECT substring('Thomas' from 2 for 3);
 substring
-----------
 hom
(1 row)
```
substring(string from pattern)

Description: Extracts substring matching POSIX regular expression. It returns the text that matches the pattern. If no match record is found, a null value is returned.

Return type: text

Example:
```
mogdb=# SELECT substring('Thomas' from '...$');
 substring
-----------
 mas
(1 row)
mogdb=# SELECT substring('foobar' from 'o(.)b');
 result
--------
 o
(1 row)
mogdb=# SELECT substring('foobar' from '(o(.)b)');
 result
--------
 oob
(1 row)
```
NOTE: If the POSIX pattern contains any parentheses, the portion of the text that matched the first parenthesized sub-expression (the one whose left parenthesis comes first) is returned. You can put parentheses around the whole expression if you want to use parentheses within it without triggering this exception.
substring(string from pattern for escape)

Description: Extracts substring matching SQL regular expression. The declared schema must match the entire data string; otherwise, the function fails and returns a null value. To indicate the part of the pattern that should be returned on success, the pattern must contain two occurrences of the escape character followed by a double quote ("). The text matching the portion of the pattern between these markers is returned.

Return type: text

Example:
```
mogdb=# SELECT substring('Thomas' from '%#"o_a#"_' for '#');
 substring
-----------
 oma
(1 row)
```
rawcat(raw,raw)

Description: Indicates the string concatenation functions.

Return type: raw

Example:
```
mogdb=# SELECT rawcat('ab','cd');
 rawcat
--------
 ABCD
(1 row)
```
regexp_like(text,text,text)

Description: Indicates the mode matching function of a regular expression.

Return type: bool

Example:
```
mogdb=# SELECT regexp_like('str','[ac]');
 regexp_like
-------------
 f
(1 row)
```
regexp_substr(text,text)

Description: Extracts substrings from a regular expression. Its function is similar to substr. When a regular expression contains multiple parallel brackets, it also needs to be processed.

Return type: text

Example:
```
mogdb=# SELECT regexp_substr('str','[ac]');
 regexp_substr
---------------

(1 row)
```
regexp_matches(string text, pattern text [, flags text])

Description: Returns all captured substrings resulting from matching a POSIX regular expression against the string. If the pattern does not match, the function returns no rows. If the pattern contains no parenthesized sub-expressions, then each row returned is a single-element text array containing the substring matching the whole pattern. If the pattern contains parenthesized sub-expressions, the function returns a text array whose _n_th element is the substring matching the _n_th parenthesized sub-expression of the pattern.

The optional flags argument contains zero or multiple single-letter flags that change function behavior. i indicates that the matching is not related to uppercase and lowercase. g indicates that each matching substring is replaced, instead of replacing only the first one.
NOTICE: If the last parameter is provided but the parameter value is an empty string (") and the SQL compatibility mode of the database is set to ORA, the returned result is an empty set. This is because the ORA compatible mode treats the empty string (") as NULL. To resolve this problem, you can:
- Change the database SQL compatibility mode to TD.
- Do not provide the last parameter or do not set the last parameter to an empty string.
Return type: setof text[]

Example:
```
mogdb=# SELECT regexp_matches('foobarbequebaz', '(bar)(beque)');
 regexp_matches
----------------
 {bar,beque}
(1 row)
mogdb=# SELECT regexp_matches('foobarbequebaz', 'barbeque');
 regexp_matches
----------------
 {barbeque}
(1 row)
mogdb=#  SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
    result
--------------
 {bar,beque}
 {bazil,barf}
(2 rows)
```
regexp_split_to_array(string text, pattern text [, flags text ])

Description: Splits string using a POSIX regular expression as the delimiter. The regexp_split_to_array function behaves the same as regexp_split_to_table, except that regexp_split_to_array returns its result as an array of text.

Return type: text[]

Example:
```
mogdb=# SELECT regexp_split_to_array('hello world', E'\\s+');
 regexp_split_to_array
-----------------------
 {hello,world}
(1 row)
```
regexp_split_to_table(string text, pattern text [, flags text])

Description: Splits string using a POSIX regular expression as the delimiter. If there is no match to the pattern, the function returns the string. If there is at least one match, for each match it returns the text from the end of the last match (or the beginning of the string) to the beginning of the match. When there are no more matches, it returns the text from the end of the last match to the end of the string.

The flags parameter is a text string containing zero or more single-letter flags that change the function's behavior. i indicates that the matching is not related to uppercase and lowercase. g indicates that each matching substring is replaced, instead of replacing only the first one.

Return type: setof text

Example:
```
mogdb=# SELECT regexp_split_to_table('hello world', E'\\s+');
 regexp_split_to_table
-----------------------
 hello
 world
(2 rows)
```
repeat(string text, number int )

Description: Repeats string the specified number of times.

Return type: text

Example:
```
mogdb=# SELECT repeat('Pg', 4);
  repeat
----------
 PgPgPgPg
(1 row)
```
NOTE: The maximum size of memory allocated at a time cannot exceed 1 GB due to the memory allocation mechanism of the database. Therefore, the maximum value of number cannot exceed (1 GB - x)/lengthb (string) - 1. x indicates the header length, which is usually greater than 4 bytes. The value varies in different scenarios.
replace(string text, from text, to text)

Description: Replaces all occurrences in string of substring from with substring to.

Return type: text

Example:
```
mogdb=# SELECT replace('abcdefabcdef', 'cd', 'XXX');
    replace
----------------
 abXXXefabXXXef
(1 row)
```
reverse(str)

Description: Returns reversed string.

Return type: text

Example:
```
mogdb=# SELECT reverse('abcde');
 reverse
---------
 edcba
(1 row)
```
right(str text, n int)

Description: Returns the last n characters in a string. When n is negative, return all but first |n| characters.

Return type: text

Example:
```
mogdb=# SELECT right('abcde', 2);
 right
-------
 de
(1 row)

mogdb=# SELECT right('abcde', -2);
 right
-------
 cde
(1 row)
```
rpad(string text, length int [, fill text])

Description: Fills up the string to length length by prepending the characters fill (a space by default). If the string is already longer than length then it is truncated.

Return type: text

Example:
```
mogdb=# SELECT rpad('hi', 5, 'xy');
 rpad
-------
 hixyx
(1 row)
```
rtrim(string text [, characters text])

Description: Removes the longest string containing only characters from characters (a space by default) from the end of string.

Return type: text

Example:
```
mogdb=# SELECT rtrim('trimxxxx', 'x');
 rtrim
-------
 trim
(1 row)
```
substrb(text,int,int)

Description: Extracts a substring. The first int indicates the start position of the subtraction. The second int indicates the number of characters subtracted.

Return type: text

Example:
```
mogdb=# SELECT substrb('string',2,3);
 substrb
---------
 tri
(1 row)
```
substrb(text,int)

Description: Extracts a substring. int indicates the start position of the subtraction.

Return type: text

Example:
```
mogdb=# SELECT substrb('string',2);
 substrb
---------
 tring
(1 row)
```
substr(bytea,from,count)

Description: Extracts a substring from bytea. from specifies the position where the extraction starts. count specifies the length of the extracted substring.

Return type: text

Example:
```
mogdb=# SELECT substr('string',2,3);
 substr
--------
 tri
(1 row)
```

string || string

Description: Concatenates strings.

Return type: text

Example:

mogdb=# SELECT 'MPP'||'DB' AS RESULT;
 result
--------
 MPPDB
(1 row)

string || non-string or non-string || string

Description: Concatenates strings and non-strings.

Return type: text

Example:
```
mogdb=# SELECT 'Value: '||42 AS RESULT;
  result
-----------
 Value: 42
(1 row)
```
split_part(string text, delimiter text, field int)

Description: Splits string on delimiter and returns the fieldth column (counting from text of the first appeared delimiter).

Return type: text

Example:
```
mogdb=# SELECT split_part('abc~@~def~@~ghi', '~@~', 2);
 split_part
------------
 def
(1 row)
```
strpos(string, substring)

Description: Specifies the position of a substring. It is the same as position(substring in string). However, the parameter sequences of them are reversed.

Return type: int

Example:
```
mogdb=# SELECT strpos('source', 'rc');
 strpos
--------
      4
(1 row)
```
to_hex(number int or bigint)

Description: Converts number to a hexadecimal expression.

Return type: text

Example:
```
mogdb=# SELECT to_hex(2147483647);
  to_hex
----------
 7fffffff
(1 row)
```
translate(string text, from text, to text)

Description: Any character in string that matches a character in the from set is replaced by the corresponding character in the to set. If from is longer than to, extra characters occurred in from are removed.

Return type: text

Example:
```
mogdb=# SELECT translate('12345', '143', 'ax');
 translate
-----------
 a2x5
(1 row)
```
length(string)

Description: Obtains the number of characters in a string.

Return type: integer

Example:
```
mogdb=# SELECT length('abcd');
 length
--------
      4
(1 row)
```
lengthb(string)

Description: Obtains the number of characters in a string. The value depends on character sets (GBK and UTF8).

Return type: integer

Example:
```
mogdb=# SELECT lengthb('Chinese');
 lengthb
---------
       7
(1 row)
```
substr(string,from)

Description:

Extracts substrings from a string.

from indicates the start position of the extraction.
- If from starts at 0, the value 1 is used.
- If the value of from is positive, all characters from from to the end are extracted.
- If the value of from is negative, the last n characters in the string are extracted, in which n indicates the absolute value of from.
Return type: varchar

Example:

If the value of from is positive:
```
mogdb=# SELECT substr('ABCDEF',2);
 substr
--------
 BCDEF
(1 row)
```
If the value of from is negative:
```
mogdb=# SELECT substr('ABCDEF',-2);
 substr
--------
 EF
(1 row)
```
substr(string,from,count)

Description:

Extracts substrings from a string.

from indicates the start position of the extraction.

count indicates the length of the extracted sub-string.
- If from starts at 0, the value 1 is used.
- If the value of from is positive, extract count characters starting from from.
- If the value of from is negative, extract the last n count characters in the string, in which n indicates the absolute value of from.
- If the value of count is smaller than 1, null is returned.
Return type: varchar

Example:

If the value of from is positive:
```
mogdb=# SELECT substr('ABCDEF',2,2);
 substr
--------
 BC
(1 row)
```
If the value of from is negative:
```
mogdb=# SELECT substr('ABCDEF',-3,2);
 substr
--------
 DE
(1 row)
```
substrb(string,from)

Description: The functionality of this function is the same as that of SUBSTR(string,from). However, the calculation unit is byte.

Return type: bytea

Example:
```
mogdb=# SELECT substrb('ABCDEF',-2);
 substrb
---------
 EF
(1 row)
```
substrb(string,from,count)

Description: The functionality of this function is the same as that of SUBSTR(string,from,count). However, the calculation unit is byte.

Return type: bytea

Example:
```
mogdb=# SELECT substrb('ABCDEF',2,2);
 substrb
---------
 BC
(1 row)
```

trim([leading |trailing |both] [characters] from string)

Description: Removes the longest string containing only the characters (a space by default) from the start/end/both ends of the string.

Return type: varchar

Example:

mogdb=# SELECT trim(BOTH 'x' FROM 'xTomxx');
 btrim
-------
 Tom
(1 row)

mogdb=# SELECT trim(LEADING 'x' FROM 'xTomxx');
 ltrim
-------
 Tomxx
(1 row)

mogdb=# SELECT trim(TRAILING 'x' FROM 'xTomxx');
 rtrim
-------
 xTom
(1 row)

rtrim(string [, characters])

Description: Removes the longest string containing only characters from characters (a space by default) from the end of string.

Return type: varchar

Example:
```
mogdb=# SELECT rtrim('TRIMxxxx','x');
 rtrim
-------
 TRIM
(1 row)
```
ltrim(string [, characters])

Description: Removes the longest string containing only characters from characters (a space by default) from the start of string.

Return type: varchar

Example:
```
mogdb=# SELECT ltrim('xxxxTRIM','x');
 ltrim
-------
 TRIM
(1 row)
```
upper(string)

Description: Converts the string into the uppercase.

Return type: varchar

Example:
```
mogdb=# SELECT upper('tom');
 upper
-------
 TOM
(1 row)
```
lower(string)

Description: Converts the string into the lowercase.

Return type: varchar

Example:
```
mogdb=# SELECT lower('TOM');
 lower
-------
 tom
(1 row)
```
rpad(string varchar, length int [, fill varchar])

Description: Fills up the string to length length by prepending the characters fill (a space by default). If the string is already longer than length then it is truncated.

length in MogDB indicates the character length. One Chinese character is counted as one character.

Return type: varchar

Example:
```
mogdb=# SELECT rpad('hi',5,'xyza');
 rpad
-------
 hixyz
(1 row)
```
```
mogdb=# SELECT rpad('hi',5,'abcdefg');
 rpad
-------
 hiabc
(1 row)
```
instr(string,substring[,position,occurrence])

Description: Queries and returns the value of the substring position that occurs the occurrence (first by default) times from the position (1 by default) in the string.
- If the value of position is 0, 0 is returned.
- If the value of position is negative, searches backwards from the last _n_th character in the string, in which n indicates the absolute value of position.
In this function, the calculation unit is character. One Chinese character is one character.

Return type: integer

Example:
```
mogdb=# SELECT instr('corporate floor','or', 3);
 instr
-------
     5
(1 row)
```
```
mogdb=# SELECT instr('corporate floor','or',-3,2);
 instr
-------
     2
(1 row)
```
initcap(string)

Description: The first letter of each word in the string is converted into the uppercase and the other letters are converted into the lowercase.

Return type: text

Example:
```
mogdb=# SELECT initcap('hi THOMAS');
  initcap
-----------
 Hi Thomas
(1 row)
```
ascii(string)

Description: Indicates the ASCII code of the first character in the string.

Return type: integer

Example:
```
mogdb=# SELECT ascii('xyz');
 ascii
-------
   120
(1 row)
```
replace(string varchar, search_string varchar, replacement_string varchar)

Description: Replaces all search-string in the string with replacement_string.

Return type: varchar

Example:
```
mogdb=# SELECT replace('jack and jue','j','bl');
    replace
----------------
 black and blue
(1 row)
```
lpad(string varchar, length int[, repeat_string varchar])

Description: Adds a series of repeat_string (a space by default) on the left of the string to generate a new string with the total length of n.

If the length of the string is longer than the specified length, the function truncates the string and returns the substrings with the specified length.

Return type: varchar

Example:
```
mogdb=# SELECT lpad('PAGE 1',15,'*.');
      lpad
-----------------
 *.*.*.*.*PAGE 1
(1 row)
```
```
mogdb=# SELECT lpad('hello world',5,'abcd');
 lpad
-------
 hello
(1 row)
```

concat(str1,str2)

Description: Connects str1 and str2 and returns the string.

Return type: varchar

Example:

mogdb=# SELECT concat('Hello', ' World!');
    concat
--------------
 Hello World!
(1 row)
mogdb=# SELECT concat('Hello', NULL);
 concat
--------
 Hello
(1 row)

chr(integer)

Description: Specifies the character of the ASCII code.

Return type: varchar

Example:
```
mogdb=# SELECT chr(65);
 chr
-----
 A
(1 row)
```

regexp_substr(source_char, pattern)

Description: Extracts substrings from a regular expression.

Return type: varchar

Example:

mogdb=# SELECT regexp_substr('500 Hello World, Redwood Shores, CA', ',[^,]+,') "REGEXPR_SUBSTR";
  REGEXPR_SUBSTR
-------------------
 , Redwood Shores,
(1 row)

regexp_replace(string, pattern, replacement [,flags ])

Description: Replaces substring matching POSIX regular expression. The source string is returned unchanged if there is no match to the pattern. If there is a match, the source string is returned with the replacement string substituted for the matching substring.

The replacement string can contain \n, where n is 1 through 9, to indicate that the source substring matching the _n_th parenthesized sub-expression of the pattern should be inserted, and it can contain & to indicate that the substring matching the entire pattern should be inserted.

The optional flags argument contains zero or multiple single-letter flags that change function behavior. i indicates that the matching is not related to uppercase and lowercase. g indicates that each matching substring is replaced, instead of replacing only the first one.

Return type: varchar

Example:
```
mogdb=# SELECT regexp_replace('Thomas', '.[mN]a.', 'M');
 regexp_replace
----------------
 ThM
(1 row)
mogdb=# SELECT regexp_replace('foobarbaz','b(..)', E'X\\1Y', 'g') AS RESULT;
   result
-------------
 fooXarYXazY
(1 row)
```
concat_ws(sep text, str"any" [, str"any" [, …] ])

Description: The first parameter is used as the separator, which is associated with all following parameters.
NOTICE:
- If the first parameter value is NULL, the returned result is an empty set.
- If the first parameter value is an empty string (") and the SQL compatibility mode of the database is set to ORA, the returned result is an empty set. This is because the ORA compatible mode treats the empty string (") as NULL. To resolve this problem, you can change the database SQL compatibility mode to MY, TD or PG.
Return type: text

Example:
```
mogdb=# SELECT concat_ws(',', 'ABCDE', 2, NULL, 22);
 concat_ws
------------
 ABCDE,2,22
(1 row)
```

convert(string bytea, src_encoding name, dest_encoding name)

Description: Converts the bytea string to dest_encoding. src_encoding specifies the source code encoding. The string must be valid in this encoding.

Return type: bytea

Example:

mogdb=# SELECT convert('text_in_utf8', 'UTF8', 'GBK');
          convert
----------------------------
 \x746578745f696e5f75746638
(1 row)

NOTE: If the rule for converting between source to target encoding (for example, GBK and LATIN1) does not exist, the string is returned without conversion. See the pg_conversion system catalog for details. Example:
mogdb=# show server_encoding;
server_encoding
-----------------
LATIN1
(1 row)
mogdb=# SELECT convert_from('some text', 'GBK');
convert_from
--------------
some text
(1 row)
db_latin1=# SELECT convert_to('some text', 'GBK');
  convert_to
----------------------
\x736f6d652074657874
(1 row)
db_latin1=# SELECT convert('some text', 'GBK', 'LATIN1');
   convert
----------------------
\x736f6d652074657874
(1 row)

convert_from(string bytea, src_encoding name)

Description: Converts the long bytea using the coding mode of the database.

src_encoding specifies the source code encoding. The string must be valid in this encoding.

Return type: text

Example:
```
mogdb=# SELECT convert_from('text_in_utf8', 'UTF8');
 convert_from
--------------
 text_in_utf8
(1 row)
```

convert_to(string text, dest_encoding name)

Description: Converts string to dest_encoding.

Return type: bytea

Example:

mogdb=# SELECT convert_to('some text', 'UTF8');
      convert_to
----------------------
 \x736f6d652074657874
(1 row)

string [NOT] LIKE pattern [ESCAPE escape-character]

Description: Pattern matching function

If the pattern does not include a percentage sign (%) or an underscore (), this mode represents itself only. In this case, the behavior of LIKE is the same as the equal operator. The underscore () in the pattern matches any single character while one percentage sign (%) matches no or multiple characters.

To match with underscores (_) or percent signs (%), corresponding characters in pattern must lead escape characters. The default escape character is a backward slash () and can be specified using the ESCAPE clause. To match with escape characters, enter two escape characters.

Return type: Boolean

Example:
```
mogdb=# SELECT 'AA_BBCC' LIKE '%A@_B%' ESCAPE '@' AS RESULT;
 result
--------
 t
(1 row)
```
```
mogdb=# SELECT 'AA_BBCC' LIKE '%A@_B%' AS RESULT;
 result
--------
 f
(1 row)
```
```
mogdb=# SELECT 'AA@_BBCC' LIKE '%A@_B%' AS RESULT;
 result
--------
 t
(1 row)
```
REGEXP_LIKE(source_string, pattern [, match_parameter])

Description: Indicates the mode matching function of a regular expression.

source_string indicates the source string and pattern indicates the matching pattern of the regular expression. match_parameter indicates the matching items and the values are as follows:
- 'i': case-insensitive
- "c": case-sensitive
- "n": allowing the metacharacter "." in a regular expression to be matched with a linefeed.
- "m": allows source_string to be regarded as multiple rows.
If match_parameter is ignored, case-sensitive is enabled by default, "." is not matched with a linefeed, and source_string is regarded as a single row.

Return type: Boolean

Example:
```
mogdb=# SELECT regexp_like('ABC', '[A-Z]');
 regexp_like
-------------
 t
(1 row)
```
```
mogdb=# SELECT regexp_like('ABC', '[D-Z]');
 regexp_like
-------------
 f
(1 row)
```
```
mogdb=# SELECT regexp_like('ABC', '[a-z]','i');
 regexp_like
-------------
 t
(1 row)
```

format(formatstr text [, str"any" [, …] ])

Description: Formats a string.

Return type: text

Example:

mogdb=# SELECT format('Hello %s, %1$s', 'World');
       format
--------------------
 Hello World, World
(1 row)

md5(string)

Description: Encrypts a string in MD5 mode and returns a value in hexadecimal form.

NOTE: MD5 is insecure and is not recommended.

Return type: text

Example:
```
mogdb=# SELECT md5('ABC');
               md5
----------------------------------
 902fbdd2b1df0c4f70b4a5d23525e932
(1 row)
```
decode(string text, format text)

Description: Decodes binary data from textual representation.

Return type: bytea

Example:
```
mogdb=# SELECT decode('MTIzAAE=', 'base64');
    decode
--------------
 \x3132330001
(1 row)
```
encode(data bytea, format text)

Description: Encodes binary data into a textual representation.

Return type: text

Example:
```
mogdb=# SELECT encode(E'123\\000\\001', 'base64');
  encode
----------
 MTIzAAE=
(1 row)
```

NOTE:

a string containing newline characters, for example, a string consisting of a newline character and a space, the value of length and lengthb in MogDB is 2.

In MogDB, n in the CHAR(n) type indicates the number of characters. Therefore, for multiple-octet coded character sets, the length returned by the LENGTHB function may be longer than n.

MogDB supports four types of databases: A, B, C, and PG. If the database type is not specified, the database type is A by default. The lexical analyzer of A is different from the other three types. In A, a null character string is regarded as NULL. Therefore, when the database of type A is used, if the preceding character operation function has an empter string as a parameter, no output is displayed.For example:
  mogdb=# SELECT translate('12345','123','');
    translate
   -----------

   (1 row)
This is because the kernel will determine whether the input parameter contains NULL before calling the corresponding function for processing. If so, the corresponding function will not be called, so there will be no output. In PG type, strings are handled in the same way as mogdbQL, so you won't have this problem.

Issue