New I do not understand why the stars do not change to em? I said to find the word if it starts with * and ends with *
echo preg_replace('#\b(?<=\*)(.+)(?=\*)\b#', '<em>$1</em>', 'This text has *two* *italic* bits'); Why are only the last letters of the word displayed, not the entire word?
echo preg_replace('#\*([az])+\*#', '<em>$1</em>', 'This *text* is *italic*'); 
In the case of \b(?<=\*)(.+)(?=\*)\b asterisks are part of the preview blocks, i.e. they do not become part of the replaced string. Here you can add the fact that (.+) Is a "greedy pattern", i.e. will find everything from the first asterisk to the last, (.+?) looks preferable, as it will find the text from the first asterisk to the next to the right.
In #\*([az])+\*# you have an exciting submask that captures only one letter, the quantifier must be inside the submasks.
Use
preg_replace('#\*([az]+)\*#', '<em>$1</em>', 'This *text* is *italic*'); See the online demo .
If you need to add support for all Unicode letters:
preg_replace('#\*(\p{L}+)\*#u', '<em>$1</em>', 'This *text* is *italic*'); where \p{L} finds any Unicode letter.
If the task is to find an asterisk, after which there is a letter, and then any text that ends with a letter, after which there is an asterisk, you can use
preg_replace('#\*(\p{L}(?:.*?\p{L})?)\*#u', '<em>$1</em>', 'This *text* is *italic*'); or
preg_replace('#\*(\p{L}(?:[^*]*\p{L})?)\*#u', '<em>$1</em>', 'This *text* is *italic*'); Is there a \p{L}(?:[^*]*\p{L})? will find a letter, after which optionally 0 or more of any characters other than * follow, and again any letter. Those. there are *Да, нет* , and * \\ * will not be found.
\*([^*]+)\* will find * \\ * - Wiktor StribiżewSource: https://ru.stackoverflow.com/questions/739085/
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(.+?)- teranpreg_replace('#\*(\p{L}+)\*#u', '<em>$1</em>', 'This *text* is *italic*')- you have an exciting subtitle captures only one letter at a time, the quantifier must be inside the traces. - Wiktor Stribiżew\p{L} \p{Letter} – символы, считающиеся буквами \p{M} \p{Mark} – различные символы, существующие не самостоятельно, а лишь в сочетании с другими базовыми символами (диакритические знаки, рамки и т. д.) \p{Z} \p{Separator} – символы, выполняющие функции разделителей, но не имеющие собственного визуального представления(разнообразные пробелы и т.д.) \p{S} \p{Symbol} – различные декоративные элементы и знаки \p{N} \p{Number} – цифры \p{P} \p{Punctuation} – знаки препинания \p{C} \p{Other} – прочие символы (редко используется при работе с обычным текстом)- Edward